Dolby Digital Encoders Over the Years

[Aaron Garman]

So I'm diving into the history of encoding Dolby SRD/AC3 over the years and wondering if anyone knew what was used early on to encode the soundtracks? I've been using this as a guide:

http://www.film-tech.com/warehouse/m...CHRONOLOGY.pdf

I specifically want to know what sort of things the early encoders could or couldn't do compared to the newer encoders after the introduction of DVD and the expansion of use for AC3 for television in the late 90s. I'm also interested in process for getting the signal printed on release prints.

Thanks!

[Mitchell Dvoskin]

While probably not as detailed as what you want, this https://en.wikipedia.org/wiki/Dolby_Digital link goes into each iteration of Dolby digital.

[David Ferguson]

I'm also interested in process for getting the signal printed on release prints.

As far as I know this involves giving a Dolby engineer your audio mix, and they give you back a Dolby Printmaster file (this used to be on a Dolby MO disc, but I think this was updated so you can now get it on CompactFlash, or just the raw file), which is then used by the labs to write the Dolby Digital data blocks.

This seems to be a proprietary format, at least there's nothing online about it, and the only way seems to be to get a Dolby engineer, or Dolby approved mixing studio to produce it for you. I think this is done through the Dolby DS10 encoder, but that's just what a friend told me so don't quote me on that!

I actually emailed Dolby a while ago about this (and other things), and while they were very nice, they wouldn't tell me anything about the format because

the information that you are requesting is very much proprietary and even though the general consensus within the industry is that ‘film is dead’, we would be subject to a legal review in order to be able to release any private information, which even for a one off would be a costly and long process

.

If anyone has a Dolby Digital printmaster file lying around that they're allowed to share, I'd love to take a look at the format to see what kind of information is in there, and how/if it matches up to what's printed on the film.

---

According to this old thread, there were a few minor differences between the DA10 and DA20, although these differences aren't in the AC3 decoder, just in how the two devices work.

The DA20 (and cp500) use a Zoran ZR38500 IC for the AC3 decoding, whereas (I think - I've never seen a DA10!) the DA10 used custom Dolby DSP chips. The Zoran chip is actually the same one as used in consumer devices (ie. Dvd players) and it's datasheet talks about that sort of usage for it, not about 35mm film usage.

As well as standard AC3 decoding, the chip also supports a "user designed program in external boot-strap ROM [to] add distinctions to industry standard decoder functions". In short, this means that you can feed the chip a non standard AC3 signal, and give the chip special instructions on how to handle it. This is what (I think) is done for the DA20 and CP500, as if you inspect the signal going to the chip, isn't not quite an AC3 signal. Sadly this is as far as I got to investigating this, because the I can't find the document that tells you how to write programs for the Zoran chip (Zoran was bought out by CSR, which was then bought out by Qualcomm, and they have no information about it on their site), and therefore don't know how to go about deciphering what the 35mm specific encoding is. If anyone knows any more than this, I would love to know more!

As a sidenote, the patent on the AC3 codec expired a few years ago (2017 I think?) and the format document is online if you're brave enough to try and work out what's going on in there!

[Tom Ruff]

We would typically mix the film on 3 6-track mag film machines. The dolby encoder was in the monitor chain of the dub stage. I had its own meter bridge that sat on the console and a small control head. once we got the film mixed down to the point that everyone approved it the Dolby field engineer would come and we made the MO print master. it has both the 5.1 Dolby digital and the LT/RT Sr master. from there it goes to the camera which shoots the SD-D + DTS negative.