[mythtv-users] "Do you really need a discrete audio card ?"

[Andre]

On 29 Nov 2010, at 15:27, James Courtier-Dutton wrote:

> On 26 November 2010 08:04, Andre <mythtv-list at dinkum.org.uk> wrote:
>> 
>> Not so much at line level, it's also quite likely that the audio you are receiving has already been over some long analog line level cables, especially if we are talking about live sports it's very common to use analog because AES digital doesn't work well enough over 500m to 1km.
>> 
>> When we use analog at home it's usually those awful phono connections, high impedance (so the noise is kept at high level ;-) and unbalanced co-axial so any interference is retained! The balanced 600 ohm +10dBm (often 10 ohm drive impedance) lines that outside broadcast people use are amazingly good up to a km or two. The drive electronics are too expensive for domestic hifi and deemed irrelevant over a 1m cable.
>> 
>> I am amazed that the monstrously expensive esoteric hifi people haven't latched on to proper balanced line level XLR interconnects, to me this demonstrates that they are selling snake oil and don't really know how to do things properly. I have never seen a recording studio where "oxygen free directional" cables were used, usually standard FST everywhere except the mic cables. I think some esoteric hifi nuts would throw a wobbly if they saw the cabling most studios use to create the sound they are so carefully trying to preserve ;-)
>> 
>> 
>> 
>>> </dreaming on>
>>> But the perfect word would be: several speakers with onboard DAC and amp all plugged to the mains and get their audio from them. Probably a sync problem has to be solved but well it sounds doable.
>> 
>> No sync problem if you use a simple digital format, like AES which is almost exactly spdif with some optional extra resolution. To use these for a home surround system you would need a surround decoder which can convert AC3, DTS etc to 3xPCM spdif, probably better to have 6x spdif.
>> 
> 
> I don't know why consumer stuff does AC3 and DTS decoding in the AMP.

I think it's a cultural thing, that's what is expected so that's what sells :-(


> ADAT has been around for ages, provides 8 channels of PCM over the
> same cabling as SPDIF. The PC could then have done the decoding,


> making the AMP a lot cheaper.

So here is the real reason ;-)


> Probably for the same reason consumer stuff does not use XLR (balanced)
> If consumer applications started using professional connectors, all
> the pro stuff would fall in price drastically.

Pro's would then buy domestic kit of course, I am surprised that real top end domestic kit (stratospherically priced stuff aimed at the gold mains plug people) doesn't make that step. It wouldn't dent the pro market because it would be more expensive and the pro's wouldn't buy into the directional de-oxegenated, ley line compatible components spiel. Of course then such kit could actually be quite good as well as being outrageously designed, priced and marketed ;-) 

I was asked to fix a moderately esoteric hifi amp (name withheld to protect any unwitting proud owners) which cost as much a decent second hand car in it's day. The components used where extremely low spec. and the design was "my first amplifier" throughout, I had built far more linear designs as a kid learning electronics! The owner was extremely disappointed when the replacement components (main output transistors) were replaced for £1.50! I even uprated them for him, and replaced them all to retain "balance" and still got change from a tenner.


> Another reason is that one cannot fit XLR connectors on a PCI card,
> one has to use external breakout boxes like EMU use.
> http://www.emu.com/products/category.asp?category=505

Really I don't like anything analog being on a PCI bus or inside a PC's case, not within 3 feet ideally!

A pro supplier I sometimes use coined the expression "donkey walloper" to describe the massive ungainly breakout cables usually associated with PCI cards, I think they did it to sell more breakout boxes.


> 
> This is why I favour digital out of the PCI card, because digital
> optical connectors are small but retain quality.

The electronics are so simple that I guess it's difficult to justify a high priced card without adding all the complicated looking analog bits. One tiny chip, a couple of protection diodes, caps and an optical diode would look extremely disappointing if exposed on a PCI card, better they are surrounded by expensive looking bits or hidden away on a motherboard amongst other stuff ;-)

One pro card I used a few years ago turned out on investigation to be just a PCIe line driver and all the real electronics were in the breakout box, it was a great solution, low latency and extremely low analog noise.


> Then some external device doing the ADC.

Would be the best solution for video too, I'm fed up with buying lots of different mpeg2/4/avc decoders and using absurd HDMI cables to connect them to a display that already has a better mpeg2/4/avc decoder closely integrated inside it! I guess it was tried in the US with the mandated stb firewire interface but we all know how well that worked out!

Wow just how simple would mythtv be if it was just passing a transport stream out to a external decoder/display, manky old bit of cat5 would go for miles ;-)

Andre



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