[pvrusb2] Comparison of Video Stream Bitrates

[Mike Isely]

On Wed, 20 May 2009, Roger wrote:

> On Wed, 2009-05-20 at 14:47 -0800, Roger wrote:
> > The HDHomeRun's video stream bitrate is approximately 18,000-19,000.
> > 
> > If I'm not mistaken, the HVR-1950 stream has a higher bitrate (AKA
> > quality) doesn't it?
> > 
> > (Reason being, a smaller bitrate is easier to stream over a network?)
> 
> <shrugs> never mind I.  Think the possible bitrates are similar for both
> units.

It's not a real comparison.

To compare them fairly you have to compare them in digital mode.  In 
that case both the HVR-1950 and the HDHomeRun will just grab a bit 
stream from the RF signal.  IN DIGITAL MODE THERE IS NO CAPTURE OR 
ENCODING TO BE DONE.  The "encoding" such as it is comes from the 
transmitting station, or possibly even the station's upstream source 
depending on the program.  All that the two devices do in this case is 
grab the bits out of the carrier and pass the resulting stream to 
whatever app is reading from them.  The bit rate and quality will be 
exactly equal.

Electrically, a digital TV "capture" device is actually simpler than its 
analog version.  With an analog capture device, one must:

1. Tune the signal.
2. Demodulate it to get the analog baseband video signal
3. Do A/D conversion to capture the analog signal into a frame buffer
4. Perform encoding on the frame to generate the mpeg signal

The HVR-1950 in analog mode does all of the above.  A non-encoding 
analog capture device will do all of the above except step 4.

However a digital signal is much simpler.  Basically after step 2 
instead of baseband you get a bit stream.  So a digital capture device 
need only do:

1. Tune the signal.
2. Demodulate it to get the digital bit stream

The HVR-1950 in digital mode only needs to do the above.  The same is 
true for the HDHomeRun and for EVERY other digital capture card.  

Unlike the analog case, in this case the digital bits are not created 
locally.  Rather it's the bits themselves that are sent over the air.  
So questions of encoding quality and "capture" resolution are TOTALLY up 
the transmitting site.  Thus, so long as the bits are received 
correctly, the quality of resulting video is invariant to the digital 
"capture" device.

When the HVR-1950 is in analog mode, then questions of quality come into 
the picture.  When in analog mode it's possible to configure the capture 
resolution, the encoding bit rate, various digital filters, etc.  
Realize however that the best possible result for the HVR-1950 in analog 
mode (or any standard definition analog capture device) will never equal 
the digital signal.  This because your best case in analog mode is still 
going to be 480 lines interlaced video with no noise - but the worst 
case digital signal is actually 480 line progressive.  And it only gets 
better after line (e.g. 720 lines progressive, 1080 lines interlaced).

So if you're comparing an HDHomeRun to an HVR-1950, then they are not 
comparable at all in analog mode, and in digital mode they will always 
be exactly equivalent.

Another interesting device is the HD-PVR.  This is actually another 
analog capture device, but it can capture component video at up to 1080 
lines interlaced.  That's a fantastic capability - quality theoretically 
can equal that of an HD broadcast.  This makes the device useful in 
cases where a digital cable subscriber can't otherwise get at the 
digital bit stream: just capture the analog component output instead.  
It's effectively a hardware encoding HD capture device.

I think the "ideal" capture device would be an HVR-1950 that has an HD 
capture capability like the HD-PVR for analog (but still also have the 
analog tuner).  As far as I know no such device exists on the market.  
Given the fact that analog broadcast TV in the USA is not long for this 
world now, a device like that will probably never appear.  It would 
probably be horribly expensive anyway.

  -Mike

-- 

Mike Isely
isely @ isely (dot) net
PGP: 03 54 43 4D 75 E5 CC 92 71 16 01 E2 B5 F5 C1 E8