[mythtv-users] HDMI or DVI

Michael T. Dean mtdean at thirdcontact.com
Fri Sep 14 02:29:24 UTC 2007

On 09/13/2007 04:38 PM, Andrew Burgess wrote:
>>> digital transmission is lossless
>> Wow.  When did they implement error correction with retransmission in
>> HDMI?
> You probably know what I ment but I'll try again anyway.
> Analog degrades with any length cable. Softly and gradually.
> Digital is all or nothing.

That may be true depending on the format of the digital data being
sent.  For example, given a digital stream of MPEG-2 (i.e. HDTV over
cable or OTA), you'll quickly get to a point where there are too many
errors to decode the stream--meaning you get nothing.

>  So when the digital signal gets through its
> lossless. Analog never is. As the cable lengthens, analog gets fuzzy, 
> digital goes away abruptly.

However, DVI, HDMI, and DisplayPort transmit uncompressed digital color
information for pixels.  Therefore, when you first start to get errors
in transmissions, you'll simply see "sparklies"--pixels flashing the
wrong colors causing a sparkling effect.  As you get more errors, it
will be more and more common for neighboring pixels to have errors,
causing lines and blocks.  Eventually, you'll get too many errors to
allow decoding the signal and will end up with nothing.  So, though
there is a digital cliff, it's a very gradual slope for a cliff.

But, if you're using encrypted HDMI or DisplayPort transmission (and
you're not with Myth), the cliff may be very abrupt (as errors will
prevent decryption).

> This can work to your advantage if you need a really long out of spec
> cable. VGA at 100 feet is fuzzy but usable. Most HDMI/DVI is blank.

Not to mention the fact that VGA uses coaxial cables and
DVI/HDMI/DisplayPort don't (but should have), so they tend to have
issues controlling impedance--especially on long runs.  Basically, these
new digital connections were designed for short runs, only.


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