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Inside an AV receiver 2 - Signal processing

Article Index
Inside an AV receiver 2
Inputs and outputs
Video connections
Signal processing
Loudspeaker tuning
VHF tuner and power amps
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ti-sharc-1

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

DIGITAL SIGNAL PROCESSING

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 SCHEMES

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

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.

 

DTS ES

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.

 

 

STEREO TO SURROUND-SOUND CONVERSION

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.

 

 

loudspeakers-tuning

 

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.



 
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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