[mythtv-users] Problems with alsa & spdif & myth

[David]

John Sturgeon wrote:

> I'm completely baffled.  Where is the noise/hum coming from?  This is 
> a completely digital signal.  The amplifier does not touch the signal 
> until it is decoded.  Any 'static', or 'noise' introduced by using a 
> wire instead of fiber optics would *never* be heard by the amplifier 
> because it is not an audio input.  The 1's and 0's either get there, 
> or don't.

Take a look at
http://www.epanorama.net/documents/groundloop/index.html
or
http://www.smr-home-theatre.org/Ground-Loops/

or just Google for ground loops.

It doesn't come from the signal.
It comes from the electrical conductivity afforded by a metallic link - 
ie a non-optical SPDIF cable.

The real world isn't digital :)

> Sharp bass?  Not possible.

A thundercrack is a sharp bass noise.
A thunder roll isn't.
Basically good 'attack' makes it sharp.

If you do a fourier transform on a low frequency (actually it's not 
frequency, it's periodicity but we won't get into that here!) signal 
you'll find that they contain a variety of signals. The classic example 
is a true square wave that contains all other frequencies (hence is not 
possible to achieve in nature). The less well reproduced all these 
frequencies are when you play back the signal, the worse the sound.
Typically this happens in cheap car stereos with woolly bass lines - the 
amps just can't pull the speaker cones back and forth sharply enough.

I can hear this effect when I swap cables - come and listen next time 
you're in the south of the UK if you like :)
I don't *know* *why* it happens? digital corruption, ground loop effect? 
poor electrical/optical conversion? I dunno.

David