Inside an AV receiver 2

INSIDE STORY

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.

AV RECEIVER BLOCK DIAGRAM

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.