Which cable to make DVD-Receiver Digital Coaxial lead.
 

On Wed, 20 Aug 2003 17:02:10 GMT, "jason" wrote:

Maplin sell Digital Coaxial (Shark) which is claimed is "specifically
designed for digital audio interconnects such as those used between DVD
players and digital decoders for surround sound systems, it would be an
excellent alternative to regular 75 ohms coax where high signal quality is
essential", this cable however costs £1.99 per metre (and is out of stock!).
Is standard 75ohm low loss coax not suitable? Maplin do N12AP for £1.49 for
5metres which claims Nom. conductor area:of 0·786mm&square, and Capacitance
56pF/m, Attenuation per 10m:0·75dB at 100MHz, 2·6dB at 900MHz? Will this not
work OK?

I picked-up my new Maplin catalogue on Saturday.

"The cable core is 0.5mm diameter OFC Copper plated in 80um Silver.
The Silver plating takes advantage of a phenomenon of "skin effect".
"Skin effect" describes the tendency of ultrahigh frequencies to be
conducted on the surface of the wire as opposed to through the wire's
total profile. The lower resistivity of silver over copper ensures
optimum transmission of theses [sic] ultrahigh frequencies."
[Capitalisation as in the original.]

A quick calculation indicates the 24-bit 96kHz S/PDIF has a maximum
frequency about 10MHz - which being in the middle of the HF band is
way below UHF (ultrahigh frequency). The signal level is hundreds of
millivolts. How comes my television aerial down lead works when this
is running at around 800MHz over a some tens of metres with a signal
level of a few hundred microvolts and is analogue to boot?

Methinks this cable is snake oil and a complete waste of money.


--
Chris Isbell
Southampton
UK

Which cable to make DVD-Receiver Digital Coaxial lead.
 

On Wed, 27 Aug 2003 20:26:19 +0100, Chris Isbell wrote:
Methinks this cable is snake oil and a complete waste of money.

of course it is. Any cable that isn't shorted or open will work
*perfectly*. Digital errors aren't subtle.

Which cable to make DVD-Receiver Digital Coaxial lead.
 

Methinks this cable is snake oil and a complete waste of money.

Yup.

Which cable to make DVD-Receiver Digital Coaxial lead.
 

My only regret about snake oil is that I'm too honest to make money
out of it too...

For a short run of a few metres, just about any cable will work.
Getting one of the right impedance would be best, to stop reflections,
but SPPDIF is hardly the world's best designed electrical interface.
The connectors really are so bad that you need not worry about them.

Silver plating is a waste of time. The resistivity difference is so
small it's not going to make a blind bit of difference on a digital
signal. The SPDIF receiver won't even notice 6dB of loss.

The full bandwidth of the SPDIF signal is tens of MHz if you want nice
sharp edges. If the edges are sharper then there's more change of
getting good timing back out of the signal. It might be digital, but
the timing of the 0's and 1's will affect the final audio quality.

The biggest variable in all of this is the SPDIF receiver chip. There
are many crap ones out there. I know. I've used them. They don't
recover good timing information and so the resultant sound produced
gets degraded. I also work in the design team of a chip company that
doesn't produce a crap one...

So for ultimate audio quality, use a one-box player to avoid the SPDIF
link if you can; or find a system with a properly designed SPDIF...

Which cable to make DVD-Receiver Digital Coaxial lead.
 

On 1 Sep 2003 01:05:37 -0700, (Ian Smith) wrote:

For a short run of a few metres, just about any cable will work.

Yes, probably. However, if the cable is not well screened it might be
radiating high frequency crud that could interfere with other
equipment/cables - especially if they are poorly screened. Perhaps
some of the people here who are experts of RF might like to comment on
whether or not this is likely to be audible.

I would note that for AES/EBU digital interconnects Neutrik sell
range of XLR connectors with additional screening. (They also come
with a rather neat black and white striped ring to indicate that it is
a digital connection.) Since this is a 'professional' product it is
less likely to suffer from hi-fi snake oil. (They are also priced very
similarly to their 'analogue' XLR connectors.)

When wiring-up my system I used proper screened cable of the correct
characteristic impedance for the S/PDIF with 'digital' XLR connecters
and double-screened cable for the audio. But then I'm paranoid. ;-)


--
Chris Isbell
Southampton
UK

Which cable to make DVD-Receiver Digital Coaxial lead.
 

Chris Isbell wrote in message . ..
On 1 Sep 2003 01:05:37 -0700, (Ian Smith) wrote:

For a short run of a few metres, just about any cable will work.

Yes, probably. However, if the cable is not well screened it might be
radiating high frequency crud that could interfere with other
equipment/cables - especially if they are poorly screened. Perhaps
some of the people here who are experts of RF might like to comment on
whether or not this is likely to be audible.


When wiring-up my system I used proper screened cable of the correct
characteristic impedance for the S/PDIF with 'digital' XLR connecters
and double-screened cable for the audio. But then I'm paranoid. ;-)

You make some good points Chris. I had assumed screened cable as that
was being discussed.

Not that it'll make a huge difference in a lot of systems because they
are barely EMC-compliant. I can't have my Naim CD3.5 switched on when
I'm using the FM radio as it causes too much hiss and that's with
screened analogue connections only. No surprise to me though: there's
a mains cable coming out of the CD player with no filtering on it!

If the S/PDIF receiver was good, optical would be better still.

Regards,
Ian.