MZ, first I would like to say thank you for your detailed response to my
questions! It is much apreciated!
Okay, I did the apples to apples test as you suggested, and figured I would
relay the results to you for your own personal information... I did it two
ways, first I recorded Rosanne on nickelodeon channel 40, (analog) and then
recorded the same episode on 'nick too' channel 127 (digital). Both recordings
were at best quality, and the results were that the analog recording says it is
using 1.28 GB while the digital is using 1.03 GB. I also recorded the Office at
10:00 in the living room Tivo using the internal tuner, and recorded the same
episode at the same time in the bedroom, which is strictly using the digital
box. The results were the same, it says that the livingroom version is 1.28GB
and the bedroom 1.03GB. My assumption is that one of your theories is correct
and the digital recordings are not using as much disk space.
I have two digital boxes for my dual tuner tivo, (comcast did a promotion
offering them free for 1 year) I am just waiting on the dual IR blaster from
Ebay that will allow the Tivo to control both boxes. It looks like I'll be able
to conserve space this way in addition to being able to access digital channels
on both tuners, (which was my initial reason for wanting to hook up both
digital boxes.)
Again, thank you so much for the access to your Tivo brain!
MegaZone <[EMAIL PROTECTED]> wrote:
Once upon a time hawkins1of3 shaped the electrons to say...
> using 2.20 GBs of space. A 1 hour show in best quality on Digital
> cable is 1.50 GBs of space. A show I record on basic cable that is
> only high quality is 1.50 GB of space as well..
Size will vary by show, even on the same source. MPEG-2 encoding is
largely a differential system. Key frames are encoded in full, but
then the following frames are only the differences from the key frame,
until the next key frame. Shows that have a lot of motion, rapid
camera changes, even a lot of colors, create more differences frame to
frame, and hence more data to encode. Shows that are full of static
shows, fewer colors (lots of dark scenes), or little motion compress
much more readily.
So you can't really compare two different shows. The only really
accurate test would be to record the same episode of the same show -
one from the cable box, and one from the analog tuners. You could do
this with a cable show that airs repeatedly by going into your
Channels list and forcing a channel to the box and then the internal
tuner, respectively.
That will give you an apples to apples comparison.
> So I guess there are multiple questions I have.. First would be
> whether anyone knows why it uses less space, and second if it means it
> is lower quality, and third is whether that means I can hold more
> shows on my HD if I only recorded in digital since I am using less
> space per show in digital?
1. Well, as I said above, encoding will vary show to show, so until
you have an apples to apples comparison you can't be sure it is
really saving anything significant. However, presuming it *is*,
there are a couple of possible reasons. First, it is possible that
your cable company does digital simulcast. That's where an
'analog' channels is also broadcast in the digital spectrum.
Analog tuners (like the one in the TiVo) use the analog signal, but
cable boxes (and CableCARD tuners, like the Series3) can use the
digital signal. The digital signal is generally cleaner than the
analog signal. So the cable box tunes a better signal with a
clearer image, and, especially if you're using S-Video, feeds this
to the TiVo to encode. This image has less noise than the analog
image. The encoder doesn't know noise from real image data, so it
has to encode everything. Noise - fuzz, static, etc - creates
differences frame to frame, which get encoded as image data. Take
the same image, one with noise and one without, and the one without
needs less data to store the encoded version. Another possibility
is that the cable box is tuning the same analog source, but it has
a better tuner and/or does some kind of noise filtering or image
smoothing on the image before sending it to the TiVo - resulting in
the same thing, a less noisy signal to encode.
Note, if you're doing apples to oranges comparisons (two different
programs recorded) and one is from the internal tuner (analog) and
one is from a digital cable station (box) then the analog/digital
difference (noise) is probably the answer - along with differences
in the source. But if you record an analog show via the cable box
(which you have to force - by default the S2DT always uses the
internal tuner for analog) it may not save any space over the
internal tuner - and may even use more space. That's why I say do
an apples to apples comparison.
2. No, a smaller size doesn't necessarily mean lower quality. Given
*identical* image sources, with the same encoder, a smaller file
size DOES indicate a more lossy encoding, which generally means a
lower quality reproduction when decoded. Hence Basic looks worse
than Best. However, given the same encoder, with the same settings
(both recordings are Best), if one recording is smaller than the
other, it is most likely a difference in the source material. One
program had more data to encode than the other.
3. Given apples to apples, if the cable box recordings take up less
space than the internal recordings, then sure, using the box will
allow you to store more. But, odds are, the same analog content
recorded via the internal tuner or the box will use the same amount
of space. And using the box loses the dual-tuner aspect. Using
the box also may lower the image quality - with the intenral tuner
the channel is tuned and fed directly to the encoder. With the box
it is tuned, modulated for the appropriate output, then fed over an
analog link to the TiVo, where it is captured, then fed to the
encoder.
There are many variables.
-MZ
--
megazone-at-megazone.org http://www.MegaZone.org/ Gweep, Geek, Human, me.
http://www.TiVoLovers.com/ http://www.Eyrie-Productions.com/ -><- Hail Eris
"A little nonsense now and then, is relished by the wisest men" 508-852-2171
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