Compression Question

I am having an argument with a friend and I want you experts to chime in…

Here’s the argument…

Suppose that I am encoding two video sources in the same divx encoding (and both sources are the same time-1 hr)…except that the first source is a vhs tape source and the second is a dvd … so the first source the vhs tape goes to avi (divx) and the dvd goes to avi (divx) …

Would there be a difference in the final size of the avi from either source? Would one be larger in size than the other, and if so why?

Is this a trick ?

Depends on the bitrate used for each source…
If lower kbit/s = smaller file size
filesize = bitrate x running time

Use a bitrate calc,and see for yourself…
http://www.videohelp.com/tools/VideoHelp_Bitrate_calculator

More importantly will there be a quality discrepancy. The answer to that is yes, VHS will provide a lower quality result no matter what method you use to read from the tape.

ok, let me clarify, the bit rate will be the same between the two…

and, yes, the vhs quality will of course be lower than the dvd …

but the question is will there be a difference in the final size of the divx avi between the two?

The actual arguement between us:
I’ve seen some pretty high end dvd rips in divx avi come in at 700 mb … but my friend encoded a vhs family tape into divx avi and its the same 1 hour but it comes in at 990 mb. So, I told him that if a 700 mb divx avi - 1 hr long can fit on a cd and look so good, then your vhs 1 hr long should be even smaller or at least definitely the same 700 mb size

(in case it matters, the vhs tape goes to dv first then divx avi)

VHS input is a lower quality source than dvds. You can therefor use a lower bitrate on it and still maintain quality close to that of the source. Coming from dvd, you would use higher bitrate in your divx encoding to maintain the same (or close to the same) quality in your final product.

So, bitrate shouldn’t be the same using those two types of sources. Using more bitrate than necessary is just needless spinning of your wheels, and uses more room.

Check the resolution and bitrate of your friends encoding and compare that to the 700mb divx files you are comparing it to. Make sure you are comparing apples to apples.

I’ve seen some pretty [B]high end[/B] dvd rips in divx avi come in at 700 mb …

I haven’t, unless of course you are viewing on a 27" CRT and the running time is less than an hour.

Kerry,

Thanks for your response and that seems to agree with my position. However, this is what my friend says:

“If you have an analog (vhs) wave form e.g. audio wave from a vhs tape in order to maintain the entire waveform you’re going to have to reference the waveform more than if the audio source was already digital (dvd) because its already a digital bit stream. In an analog waveform, in order to preserve the crest and rarefactions and all the data in between then you would have to have more bits per cycle (hz) than in a digital wave. That’s why mp3 compression is lossy, because when you go to a digital wave form from an analog wave some of the original wave form is lost. So, I would assume a similar effect would happen when you’re encoding video or at least with the audio of the video which would take up more space.”

My friend also went on to disagree with your post, by saying that "Because dvd is already of a high source quality, you can get away with a lower bit rate and still preserve the quality (reasonably speaking). However, with vhs, since the source quality is far lower than dvd, in order to make that at least watchable you must use a higher bitrate -than dvd- to achieve a quality close to the source.

I think my friend is fundamentally wrong, though I dont know that much about the science behind audio/video - he has a background in audio … Thanks for your insight.

[QUOTE=holyearth;2202300]Kerry,

Thanks for your response and that seems to agree with my position. However, this is what my friend says:

“If you have an analog (vhs) wave form e.g. audio wave from a vhs tape in order to maintain the entire waveform you’re going to have to reference the waveform more than if the audio source was already digital (dvd) because its already a digital bit stream. In an analog waveform, in order to preserve the crest and rarefactions and all the data in between then you would have to have more bits per cycle (hz) than in a digital wave. That’s why mp3 compression is lossy, because when you go to a digital wave form from an analog wave some of the original wave form is lost. So, I would assume a similar effect would happen when you’re encoding video or at least with the audio of the video which would take up more space.”

My friend also went on to disagree with your post, by saying that "Because dvd is already of a high source quality, you can get away with a lower bit rate and still preserve the quality (reasonably speaking). However, with vhs, since the source quality is far lower than dvd, in order to make that at least watchable you must use a higher bitrate -than dvd- to achieve a quality close to the source.

I think my friend is fundamentally wrong, though I dont know that much about the science behind audio/video - he has a background in audio … Thanks for your insight.[/QUOTE]

I, like you, think your friend is wrong. What he says doesn’t make any sense (to me, at least). There are too many variables in your example so… it’s hard to come up with a direct, concrete, short answer, but he’s wrong:
For example, DVD video is already compressed to comply with mpg2 standards and the studios/manufacturers/technicians not always do the best job in this transfer (this is specially noticeable in DVDs from last century)