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ffrr
PostPosted: Fri May 25, 2007 5:56 am 
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Posts: 134
I am planning to change over to digital sound in my home theatre system, and so I am looking at replacing the soundcard in my myth box and implementing digital out. My TV tuner cards are both DVB-T cards, so I believe I have AC-3 sound available in my recordings (although some may only be 2 channel - depends on what the TV stations choose to transmit?)

Currently I have a Soundblaster Live Value card. I don't believe it does digital out (is this right?). Here's how it looks to linux.

Code:
 cat /proc/asound/pcm
00-03: emu10k1 : Multichannel Playback : playback 1
00-02: emu10k1 efx : Multichannel Capture/PT Playback : playback 8 : capture 1
00-01: emu10k1 mic : Mic Capture : capture 1
00-00: emu10k1 : ADC Capture/Standard PCM Playback : playback 32 : capture 1



I can buy a Soundblaster Audigy Value 7.1 and it does digital out via SP/DIF, and there's a cheap digital I/O module add-on that will do optical and coax in and out.

Will this card work well with mythtv? I have R5E50 installed at present (recent upgrade) and I'd rather not change that unless I really have to I suppose. If not this card, any recommendations that aren't too expensive?

Also, not being too familiar with digital audio yet, can I use this card without the digital I/O module. In other words, what is SP/DIF - is it different to a coax digital audio output, and can it be fed to surround sound amplifiers (which I haven't purchased yet, as I don't know what inputs I will need). My LCD TV seems to only have optical audio in.
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goofnrox
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PostPosted: Fri May 25, 2007 7:17 am 
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Location: Michigan
What ports are on the back of the card? I have a sound blaster live value also, and it does support digital output, via a coaxial cable. You need a mono 3.5mm jack to RCA plug adapter. This doesn't help you much though if your TV only has optical inputs. Converters are available, but at that point you may as well buy a newer card with optical outputs. I only use it for stereo output, so I don't know how difficult it is to setup the AC-3 pass through.

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ffrr
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PostPosted: Fri May 25, 2007 4:18 pm 
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It has 5 output jacks - black/green/pink/blue/yellow Of course, I am using the green one at present for the analog stereo out. It's just that I see no mention of IEC958, or anything digital in the list of devices.
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ffrr
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PostPosted: Fri May 25, 2007 4:31 pm 
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Just found some answers on these pages

http://www.andrewkilpatrick.org/mind/spdif/
http://www.plasma-online.de/index.html? ... _live.html


Seems the spdif on the sblive is TTL (neither coax nor optical - but can be converted) and it comes off a header connecter at the rear. Not very convenient, xo I think a new card would be much better.

So, is the SB audigy, with the digital I/O converter, a good cheap choice on a budget? Basically, has anyone used it, and was it easy to get working under linux, and specifically for mythtv?
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goofnrox
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PostPosted: Fri May 25, 2007 5:41 pm 
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The yellow connector is the digital out. The links you posted look like they were all older models without the digital output on the back. All you need is the adapter for 3.5mm jack to RCA to get basic stereo audio out. Possibly AC3 also, but I've never messed with it. Buts it's still coax, which you can't use.

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ffrr
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PostPosted: Fri May 25, 2007 5:52 pm 
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I think you are right, it's a later model. I am still researching.

One thing I note though, the spdif signals on soundblasters do not appear to be true coax level signals, they are TTL logic level. I'd prefer to feed the correct +/- 0.5 volt signal into an expensive amp when I buy one. So the decision to use the audigy with the digital I/O module is made - I then have true coax AND optical to choose from.


Code:
It should be noted that S/PDIF-outs on the Live! have a TTL signal level equal to 2.0 - 2.5 V, while many devices are designed for an S/PDIF signal of 0.5 V.


and from another page

Code:
coax

The coax interface uses 75ohm coax cable with RCA connectors. Even cheesy audio patch cables work well for transmitting S/PDIF. The signal is +/-0.5V and should be terminated with 75 ohms on the receiving end. (termination is built into coax inputs)

TOSLINK

The TOSLINK interface uses cheap mylar optical fibre cables that plug into TOSLINK modules. These modules input or output a TTL signal. Both Sharp and Toshiba make the modules. Also, stereo stores like to over-charge for these cables, selling 6 foot cables for upwards of $100. A 6 foot cable should only cost $15.

There are two styles of TOSLINK connectors. Some have the standard square TOSLINK plug, and the others have an 1/8" style plug. The latter is found on portable MD recorders, and usually doubles as an analog line input. You can get modules of either type, and you can also get cables that connect between one and the other. The 1/8" is purely for compactness in small devices. Other than the shape of the plug, it works the same way.

TTL

The electrical connections to and from TOSLINK connectors is TTL. TTL is 5V/0V on/off pulses. This type of signal is used within most digital electronic devices, (although often at lower voltages) and many simple digital logic chips are compatible with TTL. Also, TTL is found on many sound cards like the Soundblaster Live!. Creative Labs sells add-on units that feature TTL<->coax or TTL<->TOSLINK converters. You can of course build these converters yourself to save money and provide exactly the kind of connections you need.


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