[mythtv-users] HD PVR 2 and mythtv

[mythtv at phipps-hutton.freeserve.co.uk]

Quoting Devin Heitmueller <dheitmueller at kernellabs.com>:

> Hi Stephen,
>
> I've worked with a number of these products.  A few useful notes follow:
>
>> A possible alternative to an HD PVR 2 is a nice gadget that I just
>> learned about, called a digital modulator or HDMI modulator.  This
>> takes video (HDMI, component or composite) and turns it into a DVB-T
>> or ATSC stream and injects it into an RF aerial connection.  For HDMI,
>> the conversion to DVB-T/ATSC packets is apparently lossless.
>
> These are not really "lossless" in any sense of the word.  The way it
> works is it takes uncompressed HDMI in and encodes it to MPEG2/H.264
> (which is a lossy operation).  Most have configurable encoding
> bitrates similar to what you would find on an HDPVR.  Also bear in
> mind that they're almost all going to do only MPEG2 and not H.264,
> since the target audience is televisions which typically don't have
> the ability to receive H.264 broadcasts in the US (e.g. to be
> compatible with traditional 8VSB and ClearQAM broadcasts).

You are probably right in that it will just encode the incoming frames  
with some fixed set of parameters but it is possible to re-compress  
video streams that originate from MPEG or H.264 without losing any  
information. The only lossy step in encoding is the quantization  
conversion and if you know the coefficients used in the original  
compression you can reuse them and you don't get generational loss.  
You have to do that per macroblock and know where the i-frames are  
located.

I worked on an ASIC for a nameless Japanese electronics  
mega-corporation that was to be used in a video mixing desk. Some of  
the inputs would be from MPEG and H.264 streams and the aim was to  
re-encode without loss. Don't know if you can buy it but it was a  
whopper of a chip and meant for broadcasters.

Cheers,
Tim.