The Art Of The Decoder, Pt1

24 Jun

For quite some time now i’ve meant to do a basic introduction to Matrix Decoding. This isn’t going into any mathematics, or how to do, or not to do, and why (not). The main idea is to get you, the reader, up to scratch with what the decoder is, what it’s supposed to do, and anything else that pops up.

So, to start, let’s take a visit to dictionary corner:

ENCODER

An encoder is a device, circuit, transducers, software program, algorithm or person that converts information from one format or code to another, for the purposes of standardization, speed or compressions.

DECODER

A decoder is either a hardware device or piece of software that converts coded data back into its original form

So, from the above we can see that a decoder is principly a device to return a previously mathematical encoded message/signal back to its original form.

In the beginning there was…..

So, in an attempt to convince the general public the need for quadraphonic reproduction in their homes, it was decided that such signals needed to be available for playback via the recently introduced Stereophonic Long Playing record. (remember this for later!)

A brief attempt at just using Amplitude matrix encoding, where it was impossible to decoded the four channels back in the same format as they were encoded, showed that there was interest in the general public, but another method was needed to be able to return the four channels back as they were encoded.

When the PAM (Phase Amplitude Matrix) method of encoding/matrixing four audio channel audio into two, using 90 degree phase shifting, was developed by Peter Scheiber, things moved on at quite a pace. There were a number of competing PAM  systems available, but after a short while there were only two systems still being used, these being Sansui’s QS and Columbia’s SQ systems.

They were quite different in their method of encoding, to the point that they were completely incompatible with each other. But one problem they did share was poor separation between the Front and Rear channels

SANSUI QS

Also know as RM, this system utilised all four audio channels to encode them into a compatible stereo signal. This is certainly the easiest of the two systems to decode, mainly because it used all four channels in the encoding process.

COLUMBIA SQ

Invented by Ben Briar whilst working for Columbia, this system took a completely different route by only utilising the two Rear channels in the encoding process. But by doing this caused some problems, most of which were never discussed outside of the companies laboratory. Also to correctly decode the rear channels it had to be done the correct way, there was no other way of doing it,

When is a Decoder not just a Decoder?

The word Decoder has become to mean more that it was originally meant to be. At first the competing companies sold what we would now call ‘Basic Decoders’, but their performance wasn’t that good, for any of the systems. The problem was due to the low level of seperation these matrix systems gave, bepending on system and circumstances, it was between 3db and 7db. Not much, so the companies developed various methods of improving their performance, which we shall just call Logic systems.

Basically these additions to the decoder circuitry varied the four levels from the decoder at such a rate/level, to give the impression that there was a greater level of seperation than there actually was by using the information in the audio itself. Unfortunately, such systems gave variable audible performance and one annoying side-effect….’Pumping’.

Now, as we can see from the above, including the logic circuitry in with the decoder, it became more than the name suggested. I make this point for one very good reason, which is that people seem to have forgotten that any decoder for either system must include both parts, the system decoder and the logic cicuitry designed to work with the information from the decoder.

I’ll leave it there for now so you can chew over what has been put forward in this basic introduction to the world of PAM.

 

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4 Responses to “The Art Of The Decoder, Pt1”

  1. atq June 28, 2017 at 17:11 #

    Great stuff! I hope you can go into more detail in future posts.

  2. crispin14 June 24, 2017 at 22:38 #

    I’ve read that SQ’s rear encoding was designed to mimic the pickup from behind (the rear of) two microphones set at 90° from each other, a popular, “purist” recording technique. This presumably in an effort to afford good compatibility for concert / ambient recordings.

    • oxforddickie June 25, 2017 at 00:29 #

      That may well be why SQ works better for classical music than QS. The BBC tested the Ghent/SQ microphone during the 1976 (if memory serves me correctly) Proms and found it very good for such recordings. It’s a shame the microphone never went into full-time use because i’ve thought for quite some time that it could have been the SQ version of what later appeared as an Ambisonic microphone.

  3. Bruce Greenberg June 24, 2017 at 20:47 #

    Do not stop! This is great. I’ve been looking for a post somewhere that goes through the entire process without the user having to be a sound engineer to understand. Thank you!

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