Remote volume control for audio DIY

Hi there, very quickly looking at the PT2258, it's clearly a digipot (albeit a more advanced one than many because it's a 6 channel job for 5.1 applications.

The problem you're facing here is not one of electronics, but more of licencing. The PT2258 is perfect for a pre-decoded stream and that's what it's intended for, either that or a digital mixing desk where multiple channels need to be connected to a single output mixer. It's quite possible to use multiple chips like this as a master volume control and that's a lower overall part count than simple e-pots.

The problem we all have with SP/DIF and similar digital streams is that they are a bitstream. The sound is digitised during mastering and the multi-channel setup (5.1 is the simplest!) is just a continuous river of serial data with sync so that the decoder can grab on and position itself correctly to read the mess of data into something that sounds like... well, sound.

But here's where the rubber meets the tarmac - these stream data formats were created by huge companies who want their money back, principally Dolby and DTS. While you *can* encode surround sound in the analogue domain (demonstrated years ago by a British inventor) that's no good to the audio coming from Netflix, Prime, Disney, a DVD or a BluRay, etc.

The "upshot" of all of this is the chips to decode these streams are big and --- you guessed it --- expensive since they (can) do more than just decode the stream into component parts.

Not the sort of thing we can DIY, at least not unless these people release the "code" into the public domain and a big silicone firm like TI make a simple chip rather than these QIL monsters.

I did find some low-cost (ish) ProLogic boards but that's an ancient technology and I can't even recall now if that operates in the digital domain or is just a fancy-pants way of filtering a stereo signal with a "rear" channel encoded on that.

The rear channel audio on Dolby Cinema audio (the earliest one) was encoded in anti-phase on the front channels. The idea is you could (sorta) do a difference mixer like Matt's USB Microphone to cancel out the centre channel (speech) and "boost" the monophonic rear channel. The professional systems also applied a few milliseconds of delay to bypass some of the phase collisions.

The centre channel was similarly just a combo of the two front channels with some filtering to pull the speech frequencies. The Subwoofer is a simple filtered signal from the mono centre channel.

It's passable, but due to the limitations of home audio setups it's highly unpredictable as there is always some amount of bleed between the channels. Usually a lot - I know this because I developed some back in the 1980s before the digital version became affordable.

Technics SH-AC500D does this in a box but I'm unsure how many are around now - although you might find one on eBay.

I haven't found a DIY solution, presumably due to the licencing problems and not even the Chinese have managed to crank out a decent one to date.

A final alternative is to get an existing AV receiver that's busted something (it's usually the power supply) and take the decoder out of that - my Onkyo receiver has one for example - but that blew a capacitor on the (wait for it) the decoder board. A tricky repair because it's SMT and I eventually bodged an axial cap in there instead. Not pretty but it's got me over the hump until I can afford to replace it. (It's a nice looking amp but my word, those things are unreliable - even the remote control died!)

I did find this but it's out of stock and the price is in Rupees I think...

Optical DTS /DOLBY Decoder (futechaudios.com)