SPDIF to Analog Converter?

[MrAl]

Hello there,


Anyone ever design or build one of these, or alternately, know of where to get a cheap one?

This converter would take an spdif audio input (optical) and convert it to analog audio.

Alternately, take an spdif electrical (RCA jack) and convert that to analog audio.

Either?

Thank you.

[Bob Scott]

I don't know Mr. Al.
I did some reading at Wiki and found out the "S/P" stands for "Sony/Philips". Unless circumstances have changed since I worked at Sony, they do not normally supply any data of their own ICs. So I checked Philips (http://www.nxp.com) and found the UDA1355H:

http://www.nxp.com/#/pip/pip=[pip=UDA1355H_1]|pp=[t=pip,i=UDA1355H_1]
You may have to copy and paste this link to get the whole line.

http://www.nxp.com/acrobat_download/dat ... 355H_1.pdf

Maybe someone else knows if there is a prebuilt adapter available so you don't have to roll your own. This is not a simple device.

[philba]

Short answer: Sure, it's easy, you just have to provide a Dolby Digital Decoder. OK, not so easy but that's what it is used for.

Longer answer: This is used to pass digital audio to an external decoder. I know that for DVDs, it can pass Linear PCM, Dolby Digital, DTS and other encodings. I believe it is also used to pass redbook audio from CDs (the raw audio format). The stream is biphase modulated - similar to manchester encoding. In order to convert to analog, you will need to demodulate and then decode the actual stream. good luck. (might be easier to just buy a cheap AV receiver)

[haklesup]

The BOM can't be that expensive if these guys can sell one for $70 or $110
http://www.ramelectronics.net/audio-vid ... agodFBUXuQ

here is some theory
http://www.cs.utah.edu/~wingfiel/5710doc.pdf

Here are some chips that it can be implemented with
http://www.alldatasheet.co.kr/datasheet ... 26501.html
http://www.ti.com/corp/docs/landing/tas5706/?247SEM
http://www.alldatasheet.com/datasheet-p ... E5241.html

There were others but you have to read a little to weed out the ones for other digital audio formats like MPEG2 and MP3 etc. Try the search terms I used below

Lots of press on the Atmel AVR32
http://www.ecnmag.com/article.aspx?id=173844
http://www.audiodesignline.com/showArti ... =212100765

BTW, SPDIF is the coaxial RCA connector while Toslink is the optical version. AFAIK, the signals are essentially the same but one is electrical while the other is optical, conversion between the two is straightforward.

I got these googling:
digital audio decoder reference design
digital audio decoder circuit
digital audio decoder chips
digital audio decoder schematic
digital audio decoder

[MrAl]

Hi again,


Thanks for the ideas and links. It is looking like there will be a bit more to it then i care to get into
right now however so im not sure where i am going with this yet.
Every chip i've seen so far involves a micro controller too, and some programming etc.
The one complete design i did find so far involves two chips one of which i cant find for sale,
and they are both rather small pitch anyway so that makes it even harder.

Maybe something to buy that can be used out of the box instead?

Leave it to Sony / Philips to complicate a simple thing like audio. They couldnt just
leave something as simple as a headphone jack alone.

Most of the new tv's have toslink or spdif now so if you want to use an external output
with your own amplifier a converter of some type is needed. Most of the commercial
home theater systems are over $150 that come with the converter built into it.
Seems too expensive just for a simple headphone jack.

[philba]

Yeah, you aren't going to find anything off the shelf to do this for less than that.

Frankly, I don't think building a chip based system is in the realm of the hobbyist. Getting the chips can be very hard for one. getting the documentation is probably as hard or harder. Good luck on this one.

However, there is another approach that may work - Linux has all the sw needed to decode the various formats. It's all open source (just don't try to ship a product with it, though). So, if you can build a small Linux system, you could do it. In the end, the off-the-shelf AV Receiver (ebay used for cheap) will still be more cost effective.

[haklesup]

There are really 2 issues at hand here. First SPDIF and TOSLINK are hardware standards defining the physical link and electrical specs for the signals. Inside that is the type of digital modulation (actually encoding, modulation would be part of the hardware spec) you want to decode and convert to analog audio you can send to an amplifier. The chips no doubt have the cost of IP included as well as the cost to manufacture. To deliver a marketable product, there may be additional liscensing fees depending on the codec. For hobby use, you are likely free under the fair use rule to make whatever you want for yourself or prototype.

I didn't see anything cheaper than the $70 device on the ramelectronics page. I never did figure out the price for the AVR32 devel board but development boards tend to run higher than the products they spawn (i.e. an MP3 player).

[MrAl]

Hi again,

hackle:
Yes i am starting to see just how involved this kind of thing gets. I found one chip with tiny tiny pins
and it was priced at 12 dollars US and that still leaves one more to purchase so i dont know if i want
to go into this yet or not. Maybe some more reading first.

After all that there is then the question of quality. I dont know how to judge the quality of the
end product yet once it is built. I read a few complaints about some TV's that put out spdif that
have a buzz and the user does not believe it is their cable because they tried other cables.

[haklesup]

I read a few complaints about some TV's that put out spdif that
have a buzz and the user does not believe it is their cable because they tried other cables.

Since the signal is digital and needs to be demodulated then decoded to be heard on a speaker, I really doubt that the source of any buzz could be in the transmission path or the source (unless the buzz was converted from an original analog source). In most cases a buzz exists in the output or preamp stage of the final amplifier.

The person who wrote that probably doesn't have a good understanding of the difference between digital and analog. If electrical noise were to be picked up by the SPDIF cable, I would expect cutouts or clicks similar to the interference we now experience with digital video, it dosen't get fuzzy or snow anymore, it just works or it doesn't.

I suppose digital audio captured from an NTSC channel could contain undesired noise. When passed to an AV amplifier, the buzz would still be there as it was all along. The TV's internal audio amplifier could have filters that don't exist in the digital audio path particularly if the source were an analog station.

If I were going to tackle this type of project, it would be the front end for a D class amplifier, I wouldn't stop at line level analog audio though such an output may be desireable. Keep watching the Chip market, its entirely likely that a single chip solution will eventually become available as the digital AV market expands, assuming demand for one exists.

[philba]

Yes, the result of transmission errors are audible pops and/or audio dropout.

It's funny, I got into an argument with a audiophile who claimed that the quality of the cable affected the quality of the sound comping over SP/DIF. He had some wild story about how the cable induces jitter which imparts audible artifacts. Figure the jitter is on the order of 1/2 the period of the carrier and we are talking 10s of microseconds max.

[MrAl]

Hi again,

hackle:
Yes i cant wait for the single chip solutions. Surprised they are not here yet.
Im not sure i want to mess around with the two chips yet or not.
I ended up finding a TV by pure chance that has a 'regular' headphone jack which
leads to another interesting but sad story. The advertisements (all sites i could find)
all stated that this particular TV had TOSLINK output and no headphone jack.
The opposite turned out to be true: it has a headphone jack but no TOSLINK.
Good for me but not for people actually looking for TOSLINK optical output.
Another statement of how reliable information on the web can be.

philba:
I have to agree that noise can come from many places, and once the audio gets
converted (properly) to analog it has to be handled just as carefully as any other
audio. It's hard to properly assess someone else's setup from just reading their post too.

[haklesup]

I got into an argument with a audiophile who claimed that the quality of the cable affected the quality of the sound comping over SP/DIF. He had some wild story about how the cable induces jitter which imparts audible artifacts.

Audiophiles can get that way. I think it has something to do with justifying those $50 gold plated monster cables and the need to not feel foolish. I suppose that guy had little knowledge of error correction and noise rejection common with digital signals. Unfortunately, I don't have enough theory under my belt on that subject to definatively debunk his claims. Frankly I think few people over the age of 25 have the hearing range to detect such defects and few below that age that care to spend that kind of money on an audiophile setup.

[philba]

Heh heh, in the audiophile community, Monster Cable is viewed as cheap. Literally, there are people selling RCA cables for $1000. The argument goes something like this "You've just spend $20K on a state of the art audio system. Are you going to use some cheap cable to hook it up and ruin your experience?". The IWTMM says, "uh, you're right! What do you recommend?".

By the way, if anyone needs good and inexpensive cables, check out monoprice.com. I have no affiliation other than being a satisfied customer many times over.

Phil

[CeaSaR]

I was looking around at some of my older bookmarks and came across
Rod Elliot's Sound Products (ESP), an Aussie site. He has the schematics
located (>here<). I knew I had seen it somewhere.

It uses a CS8414 S/PDIF decoder and a Crystal CS4334 DAC. As he says
on that page, it is probably about the simplest way to use those chips.
AND, these are the only active devices used. Everything else is resistors
and capacitors (plus connectors, natch!).

In addition, he has the power supply, reset and auto-switching schematics.

Hope it is close to what you need.

CeaSaR

[MrAl]

Hi Ceasar,

Thanks for the link, that circuit is interesting.
The only thing i dont like is that i have read stories about the CS8414 as not being very reliable.
The CS8416 is supposed to be much better, but i dont think the pinout is the same or if it can
replace the CS8414 with some pin swapping.

Since i started this thread i ended up buying a TV that had a regular analog audio output.
What happened was the advertisement for the TV said it was SPDIF optical, but that was
not correct as when i went to the store to double check it actually had a regular analog
headphone output. This meant that i didnt need a SPDIF converter anymore.

This doesnt stop me from being a bit interested in this technology anyway though, as i
may run into this again in the future with the way the technology is changing. I'd like
to know how to build a converter by then i think.

There is one strange problem with the audio output on this TV though, and that is that there
is a 'pop' heard in the speakers (amplified speakers) when the TV is switched from
'muted' to not muted. It's quit annoying so i may have to build something to stop that next.