Inside an AV receiver 2 - Inputs and outputs

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Inside an AV receiver 2
Inputs and outputs
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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.


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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