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digital audio out: are they all equal?

G

gplanet

Senior member
i'm trying to understand if all digital audio outputs are the same or if some are better than others. For example, a computer CD/DVD drive has a digital output. is this digital output going to be exactly the same as a digital output on an expensive stand alone CD player?

also, if a soundcard has digital output and you plug the digital output of the CD drive into the card...does the signal just pass straight out through the digital out of the card? or is the card going to modify the signal, possibly converting it to analog, resampling, converting back to digital, etc.

I'd like to know because if i have an external DAC, and all digital outputs on sound cards are equal, i can spend $30 vs. a couple hundred for a sound card 🙂
 
I'm also quite curious about this question, but in a different way.

A friend of mine claims that sound card digital outs are pretty different from home-A/V digital outs, enough that they'll damage your receiver if you hook them up to them. I have a really hard time believing this, but then again this is a guy who typically does a lot of research and doesn't fall for bullsh!t when it comes to audio, so I don't know.....

But, I mean, SP/DIF is a standard, right? If it claims to fit the standard, then it must work, right?

Could you provide me with an example of a computer CD/DVD player that has a digital audio-out on it? I know that they all have ANALOG audio outs (4 wires) that hook up to the sound card, but the digital out should be the IDE cable, right? 🙂 Or maybe I'm just too far behind.... my newest CD-ROM is a 24x..... 😱 the next thing my machine gets is a DVD drive!

As far as the soundcard converting to analog and then back to digital, I REALLY can't see this happening, unless it's in a very low-end card.
 
bizmark, most modern CD-ROM type drives have a two wire digital output.

I've heard of many people hooking up the digital output on sound cards to the digital coax input on receivers without a problem, though I guess certain receivers may have issues.
 
bizmark:

I have heard that some sound cards (I believe creative labs) follow the s/pdif standard on their outputs, except that instead of switching between 0.5V and 0v, it switches between 5V and 0V.

It's entirely possible that hooking such a thing up to a receiver could damage it.

If you don't have the means to check the signal, I'd look in the manual for the card and see if they spec the output voltage on the digital out.


Now as far as the original post goes, the quality of digital outs has apparently been shown to vary somewhat - the jitter in the clock that the system uses to generate the digital signal can apparently be carried through the system and affect the output.
 
For optical signals, I don't see how it can damage your equipment since its just a light beam passing through. The receiver would just not recognize the stream and reject it. As for the coax/rca digital signals, its possible that it can damage your equipment since the signals can be at different voltage levels, but that is very unlikely since there are standards for this sort of stuff and all US manufacturers follow those standards.

Digital signals coming out of devices are pretty much the same - what matters is the receiver and how its clock-recovery circuitry is designed. But for all practical purposes, its the same signal.

As for your second question, I'm not sure what the sound card does. But I was about to design and build my own external 24-bit DAC, but I couldn't find any darn company to give me samples (or even sell me) chips that would do the dolby digital decoding.
 
blah,

check Analog Devices and Texas Instruments (Burr-Brown) out. They have samples available for many of their parts on their website - ship free next day air too! I have ordered samples for almost all the chips I need to create an external DAC.

Also, go to www.headwize.com, and head to the Library then Projects. An external DAC/headphone amplifier that is excellent is on there and includes PCB layout! I may build this next. Don't let the fact that it's "16 bit" fool you - for most products the "24 bit digital" crap is just marketing jumbo. Let your ears do the testing
 
I've checked out burr-brown for their 24-bit D/As and requested samples, but they've only sent me the $5 digital filter and NOT their $30 D/As 😀 wonder why? The problem isn't getting my hands on those D/As, I will buy them at digikey... What is a problem is that NO company will give me dolby digital decoders because I believe you need a license from dolby for that chip.

Also, if you look at burr-brown's datasheet for PCM1704, you'll see that they include a reference design which you can just use.. but it requires a decoded signal. I want to use the dolby digital encoded optical on my audigy to feed into my own designed dolby decoder with those burr brown D/As.

Which D/A chip are you using?
 
Originally posted by: blahblah99
For optical signals, I don't see how it can damage your equipment since its just a light beam passing through. The receiver would just not recognize the stream and reject it.As for your second question, I'm not sure what the sound card does. But I was about to design and build my own external 24-bit DAC, but I couldn't find any darn company to give me samples (or even sell me) chips that would do the dolby digital decoding.
Click to expand...

you could try cirrus. the licence is for royalties to dolby, so try and convince them that it is a research project for a new company and you dont have the licence yet. or find a broken av receiver and recover the chip it has, a lot of them will use ti or cirrus or arm.
 
Originally posted by: gplanet
i'm trying to understand if all digital audio outputs are the same or if some are better than others. For example, a computer CD/DVD drive has a digital output. is this digital output going to be exactly the same as a digital output on an expensive stand alone CD player? also, if a soundcard has digital output and you plug the digital output of the CD drive into the card...does the signal just pass straight out through the digital out of the card? or is the card going to modify the signal, possibly converting it to analog, resampling, converting back to digital, etc. I'd like to know because if i have an external DAC, and all digital outputs on sound cards are equal, i can spend $30 vs. a couple hundred for a sound card 🙂
Click to expand...

just get a 40usd creative 5.1 live player and use the digital out via a mono to phono converter. or check epanorama.net for a way to build a matrix board to convert the 5V signal to 1volt or there abouts.
the digital signal will probbly go through the ide cable and into the pci bus, then in creatives it is resampled to 48kHz from 44.1. Cirrus based DSP's dont resample I think and it will be done in software as far as I know, then its either driven out or dac'd and driven out.

the only diff in cdroms would be the quality of the analog signal, which is better forgotten about.
 
blah,

I will be using the AD1866N chip. You can get this DAC chip for free. It is 16-bit and good. It will sound better than most 24-bit DACs just because the DAC is designed better
 
On a similar topic....

I live in a two story house with the majority of my computers upstairs while my home theater setup is downstairs. I don't have an HTPC right now but I'd still like to run audio from one of my machines upstairs to the HT setup (music only). Since we're talking maybe 50-75 feet, it's probably not a good idea to run line-level audio from the computer to the receiver (maybe that'll work...I don't have a long enough cable to try). So I was thinking about running digital PCM instead. Some questions:

1) I assume modern DD/DTS receivers can decode a raw 2-channel PCM stream? If so, are there limitations here? I assume it's a 44.1khz stream?

2) My Denon receiver has only one digital coax input, currently used by the DVD player, and an unused optical input. I seem to have 2 options:

A) switch the DVD player to optical and run PCM over coax from the computer upstairs to the receiver.
B) continue to let the DVD player use the coax input and use an LED to convert the computer's PCM output to optical. Is this even possible?

Obviously (A) is the easiest choice but will it work? What are the cable length limitations?

 
Originally posted by: arcas
On a similar topic....

I live in a two story house with the majority of my computers upstairs while my home theater setup is downstairs. I don't have an HTPC right now but I'd still like to run audio from one of my machines upstairs to the HT setup (music only). Since we're talking maybe 50-75 feet, it's probably not a good idea to run line-level audio from the computer to the receiver (maybe that'll work...I don't have a long enough cable to try). So I was thinking about running digital PCM instead. Some questions:

1) I assume modern DD/DTS receivers can decode a raw 2-channel PCM stream? If so, are there limitations here? I assume it's a 44.1khz stream?

2) My Denon receiver has only one digital coax input, currently used by the DVD player, and an unused optical input. I seem to have 2 options:

A) switch the DVD player to optical and run PCM over coax from the computer upstairs to the receiver.
B) continue to let the DVD player use the coax input and use an LED to convert the computer's PCM output to optical. Is this even possible?

Obviously (A) is the easiest choice but will it work? What are the cable length limitations?
Click to expand...

You want to run regular copper wiring for that kind of length. I can't imagine how expensive a 50 ft optical cable would be. :Q
 
you could try it with a line of cable to see if it works but it might not drive that far as SPDIF's only supposed to be a couple of metres, also line level signals in analog will suffer from degradationa nd capacitance. You could try TOSLINK with a booster signal from sys-concept.com

alternatives are some kind of wifi based network, with a computer upstairs - a laptop or external harddisk, or my company will be bringing out a product in a short time that will function as a home optical network. It will be priced like wifi once fitting, if you wanted it, was taken into account.

The first product will be standard NTSC type svhs from the graphics card's svideo output, plus audio from the usb and two way normal USB operation, so you can use a USB based keyboard/ mouse set up to control the PC. Although there will be definition problems, on a larger TV or if I do a Media Player skin or windows XP skin with large text it shouldnt be a big problem. Also it should be possible to play computer games over the network. If the TV is HDTV I will probably try and do a DVI to HDTV version quickly as well as the design will be similar, and the USB audio may support DD surround sound, I'm not certain on that yet though.
 
Originally posted by: arcas
On a similar topic....

I live in a two story house with the majority of my computers upstairs while my home theater setup is downstairs. I don't have an HTPC right now but I'd still like to run audio from one of my machines upstairs to the HT setup (music only). Since we're talking maybe 50-75 feet, it's probably not a good idea to run line-level audio from the computer to the receiver (maybe that'll work...I don't have a long enough cable to try). So I was thinking about running digital PCM instead. Some questions:

1) I assume modern DD/DTS receivers can decode a raw 2-channel PCM stream? If so, are there limitations here? I assume it's a 44.1khz stream?

2) My Denon receiver has only one digital coax input, currently used by the DVD player, and an unused optical input. I seem to have 2 options:

A) switch the DVD player to optical and run PCM over coax from the computer upstairs to the receiver.
B) continue to let the DVD player use the coax input and use an LED to convert the computer's PCM output to optical. Is this even possible?

Obviously (A) is the easiest choice but will it work? What are the cable length limitations?
Click to expand...

1) Yes, no, and yes.
2) I'd go with the digital coax cable, and hook up a signal amplifier at about 40 feet (not sure if they sell this stuff, but you can build your own easily with a couple of inverters). The optical cable can probably reach 75ft without any problems, but that'd cost you an arm and a leg for it.


gplanet, why'd you decide to use the AD1866N? I'm planning to use the PCM1704.
 
Originally posted by: rbhawcroft
you could try it with a line of cable to see if it works but it might not drive that far as SPDIF's only supposed to be a couple of metres, also line level signals in analog will suffer from degradationa nd capacitance. You could try TOSLINK with a booster signal from sys-concept.com

alternatives are some kind of wifi based network, with a computer upstairs - a laptop or external harddisk, or my company will be bringing out a product in a short time that will function as a home optical network. It will be priced like wifi once fitting, if you wanted it, was taken into account.

The first product will be standard NTSC type svhs from the graphics card's svideo output, plus audio from the usb and two way normal USB operation, so you can use a USB based keyboard/ mouse set up to control the PC. Although there will be definition problems, on a larger TV or if I do a Media Player skin or windows XP skin with large text it shouldnt be a big problem. Also it should be possible to play computer games over the network. If the TV is HDTV I will probably try and do a DVI to HDTV version quickly as well as the design will be similar, and the USB audio may support DD surround sound, I'm not certain on that yet though.
Click to expand...

That sounds like a very interesting product! 🙂
 
Originally posted by: Sukhoi
Originally posted by: rbhawcroftyou could try it with a line of cable to see if it works but it might not drive that far as SPDIF's only supposed to be a couple of metres, also line level signals in analog will suffer from degradationa nd capacitance. You could try TOSLINK with a booster signal from sys-concept.comalternatives are some kind of wifi based network, with a computer upstairs - a laptop or external harddisk, or my company will be bringing out a product in a short time that will function as a home optical network. It will be priced like wifi once fitting, if you wanted it, was taken into account. The first product will be standard NTSC type svhs from the graphics card's svideo output, plus audio from the usb and two way normal USB operation, so you can use a USB based keyboard/ mouse set up to control the PC. Although there will be definition problems, on a larger TV or if I do a Media Player skin or windows XP skin with large text it shouldnt be a big problem. Also it should be possible to play computer games over the network. If the TV is HDTV I will probably try and do a DVI to HDTV version quickly as well as the design will be similar, and the USB audio may support DD surround sound, I'm not certain on that yet though.
Click to expand...
That sounds like a very interesting product! 🙂
Click to expand...

cool thanks. 🙂

well consider buying it or at least telling a few likely friends when it comes out in august probably.



 
blah,

I didn't create the DAC. And the PC board already exists for using the AD1866, and it has very small traces so I don't want to deal with modifying it. Also, I got the AD1866 for free.
 
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