Inside an AV receiver 2

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Inside an AV receiver 2
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Download an AV receiver manual and see if you understand it! Here's a simple guide to the technology.



AV receivers are absurdly complicated, jammed packed with amplifiers and facilities, and accompanied by arcane manuals comprising 130 pages of mostly acronyms and trade names (only Arcam supply a manual that reads normally). If you are considering getting one, or just want to know what they are about, this article covers the technological basics in an easy fashion.


AV receivers are epics of engineering many manufacturers are barely able to cope with. But they can be reduced down to a simple model, upon which the complexity is hung, as it were. Our block diagram of a 5.1 receiver shows the basics. It is divided into audio (bottom) and video (top) sections.


Although in practice video is processed in an AV receiver, they are basically an audio device that passes video through from input to output and on to the TV; some now have ‘pass through’ mode. Digital audio also passes through to the TV, but is additionally sent to internal Digital Signal Processors (DSPs) where it is furtwangled in every way imaginable, before being passed out through the Digital-to-Analogue convertors (five, in a 5.1 receiver), producing analogue sound for subsequent amplification through the power amplifiers.







Our block diagram shows the basic layout of an AV receiver and, if you follow it, helps explain their operational logic, which is a bit daunting. Their complexity comes from the many options needed to provide compatibility with legacy sources, analogue and digital, plus a web of internal cross routing and, in particular, a bewildering array of signal processing schemes that must be made available, including all Dolby, DTS and Audyssey technologies for example, plus many more described later.


The block diagram shows the wide bandwidth (370MHz), high speed HDMI digital link which carries both audio and video data and is central to modern usage. HDMI is a serial interface able to sustain very high data rates, so it can carry high definition video and audio together. It supersedes all other connection methods.


It is common for the S/PDIF inputs to be identified as ‘digital inputs’ in handbooks (they are digital audio inputs) when in fact HDMI is also a digital input. The difference is that S/PDIF is a ‘legacy’ link technology from the 1980s, of limited bandwidth and data rate, unable to carry the 24Mbps of a DTS HD Master Audio stream, for example. So it has been superseded by HDMI, which carries both audio and video at the very high data rates that high definition surround-sound and video demand.


The AV receiver is basically a digital input device, able to perform prodigious amounts of Digital Signal Processing, often within two or three powerful DSP chips. As such, analogue inputs and outputs are extras often not diligently catered for and are first victims in any cost cutting process. They are there to extend compatibility with older equipment - audio and video - but this raises price, contradicting the value of paying for such compatibility, as it may be cheaper to buy a new Blu-ray player, for example, than buy a receiver able to interface with an old player.


This also applies to video, especially where analogue S-Video, Composite and Scart connections are involved. All are outdated. Analogue input and output of video signals involves the use of costly ADCs and DACs that are peripheral to basic function of the receiver, which is to process digital audio.









This is a digital link that carries both digital audio and video. It is the most modern of all the connectors, able to carry high definition audio and video signals possessing very high data rates (24Mbps for DTS Master Audio), something older links are unable to do. HDMI in Version 1.3a and onward is compatible with DTS HD Master Audio and Dolby TrueHD, both losslessly compressed (4:1) digital audio codes commonly used on high fidelity, surround-sound movie sound tracks on Blu-ray. They may also be found on live concert recordings on Blu-ray, but here normal PCM (i.e. basic digital, uncompressed and with no check sum data) is common, when running time and the disc space requirement is less.


All modern receivers have HDMI inputs and outputs, but only recent Version 1.4a has a back channel link to convey TV sound back to the receiver, eliminating the need for a separate connection.


HDMI carries a wide variety of signalling protocols, to make TV and receiver switch on simultaneously, for example, and to help avoid ‘no sound’ scenarios. For example, a receiver unable to handle DSD code from SACD played on a Blu-ray player will signal its need for PCM, switching the player to basic PCM output (which all have). Our tests show this DSD to PCM conversion in players is performed at a sufficiently high data rate and bit depth to preserve quality.  Because receivers are built around dedicated Large Scale Integrated circuits from companies like Texas Instruments, and fabricated in the China, most commonly for Japanese brand names, what their internal processors are capable of, such as native DSD conversion, and the full extent of signalling protocols is often not fully understood by their ‘manufacturers’ we have found. This illustrates the complexity of an AV receiver.


HDMI as it currently stands is able to carry all forms of audio data from Blu-ray and DVD, both now and into the foreseeable future. Modern receivers usually have multiple switched inputs and at least two outputs, for display devices like TV. A curious omission is that of HDMI on the front panel, for camcorders; most have low quality analogue Composite video only. HDMI camcorders must be plugged in at the rear.





Our diagram shows the composition of an HDMI cable. It has 19 pins and supports both consumer electronics items and computers, being a convergent technology. Video and audio in digital form are carried by three ‘twisted pair’ lines, each able to support a massive 10.2Gbps data rate. A transmission coding scheme called TMDS is used to maximise data throughput, and audio is carried during blanking periods in the video data. The lines carry red, green and blue video signals independently, at up to 16bit colour depth. Each twisted pair is screened (yellow) to minimise crosstalk from adjacent lines and external interference, and it runs at low voltage and current, to minimise cross transmission and power consumption. The 5V supply carries just 50mA maximum. DDC carries computer data and CEC is a signalling line that allows HDMI connected components to interrogate each other. The Clock line provides a clean master clock signal for synchronisation, lessening re-clocking errors.







A digital audio link technology from the 1980s (Sony Philips Digital Interface), originally purposed for Compact Disc (i.e. two channels of PCM code at 16bit resolution and 44.1kHz sample rate, giving 1.2Mbps data rate), S/PDIF is fitted to almost all receivers and is the best way to connect old CD players.


The arrival of DVD, prior to HDMI, saw a need for this link to carry Dolby and DTS surround-sound signals. The receiver usually has to be set within its Setup menu to accept ‘PCM’ from CD and ‘Bitstream’ from DVD for surround-sound, as the compressed surround-sound data is usually termed (‘raw’ is  an alternative). Additionally, the player may have to be set to output one or the other.

To play CD through an AV receiver a CD transport can be connected via S/PDIF, and so can a Blu-ray player, for lower jitter due to the absence of video on HDMI.


S/PDIF commonly connects through an RCA phono socket input; this is an electrical link, seen as orange sockets above. A popular alternative is the optical TOSLINK (Toshiba Link), which is easy to make and carries no ground currents. However, TOSLINK demands optical transmitters and receivers and they are not all equal in quality terms. TOSLINK commonly sounds a little smoother and softer than electrical; or conversely electrical sounds better defined and more detailed. Where the option exists, experiment for the preferred sound.




Analogue audio (and video) inputs, red and white sockets shown below, must go through conversion to digital via ADCs before they can be processed. The ADCs are usually cheap, noisy and bandwidth limited to 21kHz, although also surprisingly distortion free nowadays. They overload anywhere from 1.8V to 2.2V in, below the output of many CD players. Some receivers have (red) input overload warning symbols in their displays and attenuators to prevent it, selected in the Setup menu. Tuners, CD players and Phono stages can be connected here.


Selecting Direct bypasses the input ADC and DSPs, routing the signal around to the analogue output stages. This simple option maintains analogue quality, but obviates the use of synthetic surround-sound schemes such as Dolby Pro Logic IIx or DTS Neo 6. Marantz offer Pure Direct, where all video processing is also switched off, to eliminate the high frequency interference and small amount of audible mush it produces.


Old multi-channel, analogue output SACD and DVD-A players, lacking HDMI, must connect using multi-channel ‘direct’ analogue inputs. These do not go through the DSPs either, the drawback being that loudspeaker set up within the receiver becomes unavailable. So when using these analogue inputs a player with loudspeaker tuning on-board is best.







Lowest quality but simple and functional is Composite video, carried by a single RCA phono socket, colour coded yellow (see above). Composite is basic Black and White video, with colour added, so it is Black and White compatible and will show a Black and White picture if the colour component fails for any reason. It does not offer good picture quality from most sources, being a ‘default’ option. Composite video inputs must go through conversion to digital via an ADC before passing out via HDMI. Composite with an On Screen Menu display added is not always output from a receiver as this requires an extra on-board DAC.



This is a single, small multi pin plug (see above) that carries analogue video and audio together. It was used mainly in North America but is now slowly disappearing from receivers.



The European - well, French! - alternative to the American S-Video standard, it carried a variety of analogue video and audio signals via a large, multi pin plug. It is rarely if ever used on modern receivers and is not pictured here.






This is a high quality analogue video format and still common in AV receivers, although now as a single input and single output to a TV or monitor / projector. As with the other analogue video formats, the output may not carry On Screen display setup menus. Check in the handbook before buying if you need this.




AV receivers rely on the power of modern Digital Signal Processors like the Analog Devices 'Sharc', from Texas Instruments.


The presence of powerful digital signal processors on-board AV receivers eliminates all restraints to the processing of the incoming digital audio signals. This is both a blessing and a curse - mostly the latter. Any signal processing scheme is possible, and a lot of what goes on is of arguable value. All are sold under license, adding to the cost of a receiver, and for high quality audio most are unwanted, making them a needless expense. Home Cinema is a gadget count driven market, unused to hi-fidelity minimalism, and the ability of DSPs to process audio is heavily exploited.


Quality reproduction of music is best served by setting the processing schemes aimed at Home Cinema use to Off. Honourable exceptions are Dolby ProLogic IIx and DTS Neo 6. Both convert stereo to surround-sound. With some older recordings possessing a lot of out-of-phase frontal information they can clean up the front sound stage nicely, but the effect is recording dependent. Passing TV sound through these processes can put sounds at rear too. The DSPs also process proprietary codings such as Dolby TrueHD and DTS HD Master Audio, which are a ‘must have’.


Dolby Laboratories was founded in 1965 to sell Dolby A noise reduction and in 2010 lead the field in sound processing. The Dolby AC3 surround-sound coding scheme was mandatory on DVD. Digital Theatre Systems (DTS) produced a rival in 1991 using less compression to achieve better sound quality. It was ‘extensible’ too, so could be upgraded by addition to the core data. Other companies, notably THX, Neural and Audyssey have since entered the market with their own digital signal processing schemes. Because each company has many different technologies, the list now is a long one. Our guide focuses on those useful to high quality audio reproduction through an AV receiver. Here is a list of what is important and what is not.




Compression reduces the amount of data stored and transmitted by stripping out what is deemed ‘inaudible’ in music and speech. Guided by psycho acoustic principles, at the end of the day how well these schemes work is a subjective assessment, not a scientific one. The developers must sit in front of loudspeakers to decide whether what they have done is audible or not. That is a very fallible process.


Early systems like AC3 and MP20 were comparatively crude, typically introducing general diffuseness, homogenisation of violins, softening of transients, and such like. But they had to compress a lot of data into a small space, reducing files down to one-tenth their original size. Recent compression schemes, like Dolby Digital Plus are more sophisticated and less audible in action.


Libraries of National Reference do not like to archive data-reduced material and data-reduction (‘compression’) becomes less necessary as storage volumes and digital transmission data rates increase. So compression schemes are a child of their time and, ultimately, will fade out of widespread use. From the 10:1 common with DVD, Blu-ray uses 4:1 with movie audio, or no compression at all with audio unaccompanied by video.



Dolby Surround-Sound (AC3)

The first digital compression scheme for 5.1 channel surround sound, using a high 10:1 compression ratio to produce a low data rate of 640kbps and low file size suited to the limited capacity of  DVD discs. A cinema sound track technology, AC3 adds a soft, diffuse quality to music, but is relatively elegant and easy on the ear as early compression schemes go (unlike MP20 for example). It is always provided on receivers and needed for compatibility.


Dolby Digital Plus

This is an upgrade of standard Dolby Digital AC3, with improved compression methods, increased channels, a scalable data rate etc.  Wikipedia say it isn’t compatible with old AC3, Dolby says it is! The BBC say they will use it when broadcasting surround-sound - an example of future usage.

Like many Dolby technologies it is commonly provided in modern AV receivers, but not commonly used due to the high storage capacity of Blu-ray and transmission bandwidth / data rate of HDMI.


Dolby TrueHD

This is the audiophile version of Dolby Digital. TrueHD offers lossless packing, so the audio signal is unaltered (i.e. not compressed). Derived from Meridian Lossless Packing as used on DVD-A, it decreases storage capacity (4:1 Dolby say) and data rate, and also possesses a data integrity check. It supports up to 24/192 code.

TrueHD is needed for high quality audio sound in movies, where (HD) video content consumes space. Blu-ray and HDMI can cope with raw PCM at 24/192 in six channels, however, so music discs without video (e.g. 2L Blu-ray music discs) may not use it.



DTS was an alternative technology to Dolby Digital (AC3) purposed to provide better sound quality. It can run at a higher bit rate and carry more data, but it also uses different coding technologies, and is interesting as a digital code scheme that can be ‘upgraded’, retaining the notion of backward compatibility that both VHF radio and colour television managed by transmitting a colour chrominance signal alongside a basic Black and White (B&W) luminance signal so old B&W sets could get a mono picture and new fangled colour sets a colour one. DTS call this core+extension; it would have suited DAB broadcasting, better than Neolithic MP2 with its fixed parameters.


DTS is basically a high quality,extensible digital compression scheme that found widespread acceptance in DVD and now Blu-ray. It usually carries 5.1 channels, compressed, just like Dolby Digital, but may offer better sound quality. Whether it does or not in practice depends on the original coding and it is best to listen and see, if both DTS and Dolby versions are on offer.

There is no update to DTS, as there was to Dolby Surround-sound with Dolby Digital Plus.



This is a way of getting 6.1 channels, either through matrix addition of the back Surround channels, or by discrete channels in the original recording (e.g. ‘5:10 to Yuma’).


DTS HD & HD Master Audio

DTS HD is not commonly used, but HD Master Audio is used in Fox (aka 20th Century Fox) film sound tracks and in music Blu-rays from 2L, for example. It runs at a very high bit rate, up to 24Mbps, and offers lossless coding at up to 24bit resolution and 192kHz sample rate, over any number of channels (six on Blu-ray). On Blu-ray, with its ample storage capacity and data rate, DTS HD Master Audio is as popular, or more so, than Dolby TrueHD, so it is a ‘must have’ in any receiver.

Both HD Master Audio and TrueHD are only supported by HDMI Version 1.3a onward though, and receivers prior to 2008 do not have it. It is a core+extension system, where a lossy core is accompanied by a difference signal, between the original input and the lossy core. Hi-Fi World’s comparative listening tests show there is little between original 24/192 PCM and both DTS HD Master Audio and Dolby TrueHD losslessly packed. If anything, to date, on 2L titles where the options exist, HD Master Audio is a trifle smoother and fuller bodied than TrueHD, but since it is also smoother and fuller bodied than the basic PCM code this may be a characteristic of the complex coding scheme or just the performance of current coders / decoders. Differences are very slight however.




It is possible to derive surround-sound from stereo. This is an old idea that predates AV. Doing so puts music from CD, TV and radio – and even LP – into all channels, making for a more enveloping sound. How successful it is depends upon the out-of-phase content of the original recorded material. Older recordings on CD commonly are less ‘controlled’ during the record process in this respect and  respond better to such matrix processing. Television sound and commercials also respond well. LP  does not respond well and is usually unconvincing, but SQ and QS matrix encoded Quadraphonic LPs from the 1970s may well benefit.

With matrix derived surround-sound the difference between the front Left and Right channels (first derived by connecting a loudspeaker across Left and Right channel positive loudspeaker terminals and placing it behind the listener!), is used for rears, and the Left and Right sum (mono) is directed to front Centre. Dolby Pro Logic II takes this basic idea a lot further, as their White Paper describes, to give a subtle form of surround-sound that can, interestingly, clean up the frontal sound stage of CDs, bringing improved clarity.


Dolby Pro Logic II, IIx and IIz

Dolby Pro Logic and Pro Logic II was succeeded by improved IIx and in 2009 by IIz, a scheme that ambitiously attempts to derive frontal height information from stereo and needs wall mounted loudspeakers. Dolby Pro Logic is fitted to most AV receivers and is worth having.


DTS Neo 6

This is DTS’s answer to Dolby Pro Logic. Neo 6 is more emphatic and less subtle in action, and it possesses a number of user adjustments too. Neo6 is fitted to most AV receivers, alongside Pro Logic and is an interesting alternative.





Differing distances between loudspeakers and listener produce significant arrival time differences. In the example above the arrival time difference is 27-3.3mS = 23.7mS. Loudspeaker tuning  provides compensation. It also sets level to account for differing loudspeaker sensitivities and attenuation. Loudspeaker tuning can be performed manually or automatically in most modern receivers.



Premiere in this role within modern AV receivers is the Audyssey loudspeaker tuning scheme, that equalises levels and applies distance compensation. It works well. Audyssey have since introduced additional DSP based processing schemes claimed to improve sound quality in their MultiEQ XT room correction system.


Most receivers also have on board artificial acoustic effects such as Hall, Club, Church and even specific venues such as, from Yamaha: Roxy Club (L.A.), Hall in Vienna, Hall in Munich. Yamaha collected this acoustic data in the 1980s, so you won’t find modern venues like the 02, or halls once behind the Iron Curtain, like the Mariinsky Theatre!


So called ‘loudspeaker tuning’ can mean anything and here the power of modern DSPs is used, or misused, to apply a wide variety of effects, much like graphic equalisers of yore. Our listening tests show that they can enhance some recordings, but not others and are a blunt instrument that are not worth the effort of use (selection can be tedious). Importantly, none of these tuning systems compensate for low frequency room modes, our measurements show, so bass quality and problems like ‘room boom’ are not counteracted. Direct and Pure Direct bypass options remove all processing and consistently offer the most natural sound.


Audyssey loudspeaker tuning

Using a small microphone at the listening position, connected to the receiver by a long lead, Audyssey adjusts the loudspeakers of a surround-sound system, equalising their level at the listening position and applying time compensation for distance. It also senses the presence, or not, of a subwoofer and whether to apply bass management.

Our repeated use of Audyssey fitted to receivers on review show it works well. However, it makes the Surround loudspeakers ‘obvious’, swinging attention to the rear channels. Whilst we check Audyssey tuning effectiveness, we prefer to tune manually, using our ears, not even our Bruel & Kjaer SPL meter.


Audyssey MultiEQ XT

This system Audyssey claim “removes the distortion caused by room acoustics”, working in both time and frequency domains. Our listening rooms are fully analysed, treated and well balanced. The Audyssey MutliEQ fitted to an Onkyo TX-NR1007 receiver raised midband energy, rolled off upper treble and had no affect at low frequencies, our measurements showed (long term averaging using a third octave analyser, of a music sequence). Listening correlated well with measurement: the sound was apparently clearer and more forward, but it wasn’t even or balanced and with some recordings sounded too shouty and aggressive. The immediate apparent effect was seemingly one of improvement, but over a longer term the unbalance became obvious. Direct or Pure Direct was an easier and more fulfilling listen. Audyssey MultiEQ XT is best suited to cinema dialogue, where a midband lift will enhance intelligibility.




Buried in the menus, most receivers have bass and treble tone controls and seven band graphic equalisers. Small amounts (up to 3dB) of boost or cut may well help correct loudspeaker imbalances, but room effects are not usually so amenable to such simple adjustments. At low frequencies there is too little frequency band resolution to have any useful impact on room modes, which are high Q and cover a narrow band of frequencies. Currently, this is a missed opportunity in AV receivers.




Graphic equaliser menu of a Marantz SR8002 receiver, showing nine band equaliser. The 63Hz band is set to +1dB to lift bass a little.




Most receivers have a VHF/FM and AM Medium Wave tuner. Some come with Internet radio,  DAB (Digital Audio Broadcasting) and Sirius satellite radio (USA). These are conventional tuner modules and work in the same way as any tuner, giving stereo output. It can be processed to surround-sound if wished, using Dolby Pro Logic or DTS Neo 6. DAB and Sirius compress the audio signal to reduce data rate. DAB offers more variety than VHF/FM in the UK, but the latter offers better quality if an aerial is used to avoid hiss. Internet radio offers mediocre quality from (mostly) MP3 compressed files at low data rates, but internet connected stations from around the world (circa 14000) become available.


DAB remains unavailable in many UK regions and even in London may need a dedicated DAB aerial, because obstruction by local buildings and hills substantially reduces signal strength, due to the high carrier frequency used. Basements are DAB black spots too.


Surround-sound radio and television broadcasts are available from Freesat in the UK, but a quality Freesat receiver is needed (see Humax). Connection to a receiver is via S/PDIF. Programmes are coded in Dolby Digital (AC3) or Dolby Digital Plus. Only a limited number of programmes, like music concerts and nature programmes, use surround-sound because it does not suit situations where forward facing directional microphones are used to minimise unwanted background noise.







Modern receivers are fitted with seven on-board amplifiers, usually each of 100 Watts into 8 Ohms and more – around 140 Watts – into 4 Ohms, one channel driven, our measurements show. The bigger receivers manage 120 Watts or so, but have better regulation from bigger mains transformers and typically will produce 160 Watts or so into 4 Ohms. Rarely will all channels be required to deliver full power in use, so the issue of power output  under these conditions, when the power supply would be severely loaded, is academic. With reasonably sensitive hi-fi loudspeakers (87dB SPL from 1W) the 700 Watts available produces very high volume and, unlike a stereo system, the sound fills the room evenly. A perceptible benefit is that with this power distributed among five to seven loudspeakers, each is less stressed, giving surround-sound a sense of easy dynamism.


Manufacturers like Onkyo and Marantz suppress low level non-linearity by biasing their output stages substantially into Class A in the usual Class A/B configuration and, over seven channels, this makes the receiver run hot. Cooler running receivers may seem preferable but sound quality suffers, our listening tests show. Whilst this would seem to be an ideal role for ‘digital’ switching amplifiers, they are now not used, a major U.S. manufacturer telling us failure rates were unacceptably high.


The distortion characteristics of high quality AV receivers are nowadays (2010) similar to those of hi-fi amplifiers (e.g. <0.1% thd at 10kHz, 1W), although not up with the best. Good quality AV receivers (e.g. Marantz) sound very good, on par with stereo hi-fi amplifiers. Onkyo are in the race too. Unfortunately, most other brands seem unable to meet hi-fi standards and their amplifiers sound coarse, dimensionally flat, papery and somewhat crude against better hi-fi components. Much of this is  attributable to poor component quality; every part must be ultra-cheap and this is no way to produce a good sounding amplifier.


AV receivers are built to a tight budget and their loudspeaker terminals are usually wobbly, close together and difficult to use (see above). In this environment 4mm plugs make a lot of sense, and it is best to put ID tags onto the many leads from the loudspeakers. Onkyo use a neat colour coding system that other manufacturers seem unwilling to adopt. They supply colour stickers that attach to the speaker cables and match colour coded terminals, a great help in the spaghetti junction that is the back of an AV receiver. But Onkyo also fit safety inserts that are difficult to remove and they don’t explain in their user handbooks how to remove these plugs; a self tapping screw must be used to haul them out. The safety plugs in other receivers are either non-removable or can be flipped out with a sharp knife or screwdriver, so 4mm plugs can be used.


Our block diagram shows that the 7.1 inputs run directly through the volume control and into the analogue output stages, missing the DSP, so loudspeaker distance and level compensation and all other processing is unavailable. If a multi-channel player is connected this way, perhaps an old SACD player, then ideally it needs loudspeaker compensation on-board.


Although complex and difficult to operate AV receivers can provide excellent results from a wide variety of sources, including the internet and even LP. The technology is daunting but the results – in the best products – are outstanding.

Comments (1)
AV receiver technology guide
1Thursday, 21 July 2011 09:06
Ian Walker
Very easy to understand & well written.

Look forward to similar articles on other subjects.

Ian Walker

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