[ProAudio] Looking to decode Dolby AC-2

[Richard L. Hess]

Hi, Tim,

Ah, another difficult-to-deal-with format. Someone gave me a wooden 
carving of a person sitting in the lotus position with his head in his 
hands...and I was told that was me when another format was announced.

I kind-of knew what AC-2 and AC-3 are/were, but did a little bit of 
research. By way of introduction (from 
https://www.sweetwater.com/insync/ac-2/ ),

> Another of Dolby Labs’ <https://www.sweetwater.com/insync/dolby-2/> 
> sound encoding schemes. Dolby AC-2 is a perceptually based adaptive 
> transform coding algorithm that combines very high audio quality with 
> a low bit rate, thus substantially reducing the data capacity required 
> in such applications as satellite and terrestrial links and digital 
> audio storage media. The digital algorithm developed by Dolby uses a 
> multi-band approach to take advantage of psychoacoustic masking. A bit 
> allocation scheme based on 80% fixed allocation and 20% adaptive 
> allocation keeps the complexity of the codec relatively low. Dolby 
> both manufactures professional codecs incorporating AC-2 (such as 
> Dolby FAX) and licenses the technology to other manufacturers for 
> inclusion in their products.
Googling Dolby FAX led me to the manual for the DP503 which I found here
https://www.manualslib.com/manual/40630/Dolby-Laboratories-Dp503.html?page=17
(the entire manual is available for download)
I have it here:
https://www.dropbox.com/s/rn73knm5syogsfh/dp503.pdf?dl=0
and I already had the datasheet here:
https://www.dropbox.com/s/3pwnfxmr4m52g3l/503brochure.pdf?dl=0

Alas, the DP503 is the encoder, but there is a lot of information in 
this. The data sheet says the Dolby DP524 is the corresponding decoder.
I have the datasheet here:
https://www.dropbox.com/s/76oxnm0e3xke725/524brochure.pdf?dl=0
Note that the Dolby products only work at 48 ks/s in AC-2. I don't know 
if the ISDN applications used different sampling frequencies. (below 
might help).
Manualslib.com also had the 524 manual
https://www.dropbox.com/s/jtvkufkkyliw4et/dp524.pdf?dl=0

This site appears to have one in stock:
https://www.broadcaststore.com/store/model_detail.cfm?id=797496
and there is one on eBay for $25
https://www.ebay.com/p/2263853666

I think this was one of many systems used mostly for ISDN. I wonder if 
there are software decoders available--if so, they might only run on old 
systems, but this is going back 25 or more years as I understand it.

There is an interesting article on ISDN systems from 1997. This URL is 
the first page of the article, but you can munge the URL to see 
additional pages.
https://worldradiohistory.com/hd2/IDX-Audio/Archive-Studio-Sound-IDX/IDX/90s/Studio-Sound-1997-03-OCR-Page-0093.pdf

AC-2 was rapidly replaced with "Dolby Digital" originally known as AC-3.

I was never deeply involved with these systems other than at a high level.

What was the library system that used it?

Cheers,

Richard



On 2021-12-09 1:04 p.m., Tim Požar via ProAudio wrote:
> Gang...  I am trying to help a station migrate an automation system 
> library. Alas, it is encoded in Dolby AC-2. I can't find a way to 
> convert AC-2 into something more standard. Any pointers to programs / 
> libraries / plug-ins that can support decoding Dolby AC-2 into PCM or 
> something else? Ideally something that I can script up to process 
> 6,000 or so cuts.
>
> Thanks in advance.
> Tim
> _______________________________________________
> ProAudio mailing list
> ProAudio at bach.pgm.com
> http://bach.pgm.com/mailman/listinfo/proaudio

-- 
Richard L. Hess                   email:richard at richardhess.com
Aurora, Ontario, Canadahttp://www.richardhess.com/
http://www.richardhess.com/tape/contact.htm  
Quality tape transfers -- even from hard-to-play tapes.
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