Saturday, December 12, 2015

SQ Quadraphonic - A Dead Surround Audio Format.... or is it?

The early 70s introduced the war of Quadraphonic formats that included SQ, QS, CD-4 and others. This was back when Hi-Fi was a thing, and was something a reasonable number of people had, even if only stereo systems.

None of those formats succeeded at that time and quadraphonic died out.  All the systems had problems and the early SQ and QS formats only gave an almost inaudible (3dB) front to rear channel separation in addition to costing twice as much... and the last thing a Classical record needs is to put the audience in the middle of the orchestra.  Rock didn't fair much better.

Later Full Logic decoders appeared.  These improved the channel separation  by using gain elements in the 4 output channels to exaggerate the difference between the channels in a controlled way to improve the channel separation.  Didn't help the market grow though.

SQ Full Logic Block Diagram

I have the 12" Vinyl Deep Purple Machine Head album emblazoned with a SQ logo and QUADRAPHIC, and have never heard it in 4 channels.  Been curious about that.

Now days every Home Cinema has Dolby Surround, and Cinemas are now going to Dolby's ATMOS system with some 60 channels.  But this cinema stuff doesn't really fit music, which doesn't have or want a centre channel, or delayed rear channels.

So could you use a Dolby Surround system with this old SQ Quadraphonic stuff?   That would  require some significant signal processing on the SQ encoded stereo signal first so your Dolby decoder only produced the front left and right and musical rear signals.  Even then, the rear channels were originally restricted to only 100Hz to 7kHz and had a 25ms or so delay added to them. Lots of compromises required, and the result wouldn't be what was mastered on the album.

Most people don't seem to care to sit and listen to music as they once did, unless it is on headphones from an MP3 Player,  so it would seem investigating and even SQ encoding our own musical endeavours in a dead quadraphonic system  is pretty much a waste of time and effort. 

That maybe, but we are considering it as we are pretty much set up to do it anyway with the Reaper DAW and it's JS development language and support for the encoder, and a switchable to  4 speaker and amplifier mixing environment.

The SQ Decoder, and doing that with a few transistors and a SMD based PCB for fabrication by OSH Park is not out of the question. either.  Not a huge Altium Project by any means.  

SQ Decoder Schematic

So not out of the question.   And these things from places like Sony still show up on eBay too.  A Development Kit with fast MCU and Stereo IN, Quad OUT, 16bit+ codecs would be a flexible approach for us C developers too.

So why?   But what about a music with visuals, as in a video?

Dolby Surround (Pro Logic) is a simple system, and what was used on the first Star Wars film in 1977. Their first Surround Decoders where actually designed by the people that did the best SQ decoders. What that system does give you is great separation between the front centre and mono rear surround speakers. Fine if you want Ambience in the rear channels. Not unworkable.

Now the original standard decoder filtered the Surround channel from 10Hz to 7.5kHz. Seems the later "Music" support of Dolby surround now actually has 5 equal full range speakers.
So a quick test of Surround  as L-R, with buffered inputs, and a L+R Center channel, and a level control on the front channels using dual LM833 Opamps looks like this when prototyped quickly. It is powered from a regulated 12VDC source.  The front channels are our standard mix set up, the rear are a DELL satellite + sub woofer we usually use for background music.

Surround = L -R, Centre = L+R matrix decoder
The schematic with a later added bypass switch, to disable the surround mix system is:

In Reaper, our music production DAW, we generated a surround channel by sending the rear tracks, that now aren't routed to mix output to Surround L and Surround R, with Surround L  inverted, and then add these to the mix.  So that tracks that are not dead Center or Rear are a bit of front and rear, and surround tracks are just from the rear. Dead Center just from the front.
The result sounds interesting, and stereo compatible, but not mono compatible. We will try a HD AUDIO RUSH decoder soon, and plan to do our mixes with these rear channels as one mix option.

It is surprising what a Mixed Media Concept Album can inspire in you.  I've spent the last couple of weeks listening to, and reading The Pentateuch of the Cosmogony

Now that isn't in surround, but has given us the drive to expand our own mixed media music art tech project....  An extension of our  SteamPunk Sunset.

.... unless life gets in the way, or something else comes up of course.

So SQ might be dead,  but surround and matrix encoding and decoding that came from it aren't.

And maybe this is a more appropriate logo for what we have done so far......


Tom Carlson said...

Fascinating stuff! Really appreciate the insights on this blog. Thank you & please keep it up

Glenn Ficarra said...

Did the SQ board ever come to fruition? Looks like a fun project for listening to some of my SQ LPs.
would love to know if there are further revisions and what the transistors are...

Glenn Ficarra said...
This comment has been removed by a blog administrator.
Adrian Bruce said...

I didn't make the PCB, just formalized the veroboard unit I show in the blog. This video has the SQ soundtrack:
The only thing put into the rear channels are the "alien whale call" Theremin tones. This is the first thing you hear. When you play this back in stereo, they have a strange "other" placement, but with my decode, they just came out the rear channels..