What the Beeb sounds like straight off the SN76489!..

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BrokenARM
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What the Beeb sounds like straight off the SN76489!..

Post by BrokenARM » Mon Apr 15, 2019 1:29 pm

I recently retrofitted three BBCs with direct audio outputs straight off pin 7 of the sound chip via a 1uF metal film coupling capacitor and I have to say I have been quite surprised just how quiet (noise free) the raw audio from the SN76489AN actually is - it seems primarily the noise is getting into the audio buffer/mixer/filter and PA stages! (which, I have to say, rather too reminds me of the stock output stages of the 300/400 series Archimedes audio... grumble grumble..)

I recently made these recordings of the superb new Beeb Tracker directly from the SN76489 output pin as an experiment: :D

https://soundcloud.com/hideaway-studio/ ... ro-sn76489
Last edited by BrokenARM on Mon Apr 15, 2019 3:09 pm, edited 1 time in total.

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1024MAK
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Re: What the Beeb sounds like straight off the SN76489!..

Post by 1024MAK » Mon Apr 15, 2019 2:48 pm

Yeah, part of the problem with the audio / sound circuitry in the Beeb, is it picks up signals from the digital circuitry, hence the noise changes depending on what the CPU and the display system is doing...

Acorn did try to filter the +5V and the -5V supply rails to the audio / sound circuitry with 10 ohm resistors and 10uF capacitors, but that alone is not enough. Really to minimise noise pickup from the digital circuitry, the audio / sound circuitry needs it’s own 0V ground system (a star ‘earth’ ground system), much better supply rail filtering (or better yet, completely separate supply rails) and to be on a corner of the board far away from the high speed digital circuitry with a guard ground rail track. And last, but not least, no long audio input tracks winding their way across the board to an expansion bus (1MHz bus) that acts as a receiving aerial...

But then, Acorn were not alone in making this kind of mistake. Other home computer manufacturers made similar mistakes with analogue and digital circuitry affecting one another.

Mark

BrokenARM
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Re: What the Beeb sounds like straight off the SN76489!..

Post by BrokenARM » Mon Apr 15, 2019 3:05 pm

1024MAK wrote:
Mon Apr 15, 2019 2:48 pm
Yeah, part of the problem with the audio / sound circuitry in the Beeb, is it picks up signals from the digital circuitry, hence the noise changes depending on what the CPU and the display system is doing...

Acorn did try to filter the +5V and the -5V supply rails to the audio / sound circuitry with 10 ohm resistors and 10uF capacitors, but that alone is not enough. Really to minimise noise pickup from the digital circuitry, the audio / sound circuitry needs it’s own 0V ground system (a star ‘earth’ ground system), much better supply rail filtering (or better yet, completely separate supply rails) and to be on a corner of the board far away from the high speed digital circuitry with a guard ground rail track. And last, but not least, no long audio input tracks winding their way across the board to an expansion bus (1MHz bus) that acts as a receiving aerial...

But then, Acorn were not alone in making this kind of mistake. Other home computer manufacturers made similar mistakes with analogue and digital circuitry affecting one another.

Mark
Rather than trying to clean up the supply rails in a single ended design, another approach is to employ a ground sensing circuit and use common-mode rejection to remove it with a differential amp and/or provide a virtual ground (a common trick in hybrid audio designs) - in fact Hugo used a similar trick in his infamous Empeg In Car player to overcome the same sort of issue and it was extremely effective. Trouble was, in the latter case, you had to make sure all of the wiring to the amps was floating and not chassis referenced otherwise all the noise came back in bucket loads! :D

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