[geeks] Whee! Lightning strikes, AGAIN!

[gsm at mendelson.com]

On Wed, Jul 29, 2009 at 10:22:17AM -0400, Joshua Boyd wrote:

>Not quite.  The chroma sampling is often reduced, so while each pixel
>has its own luma sample, adjacent pixels may share chroma samples.
>4:2:2, which is NTSC, uses half the chroma samples.  4:1:1 and 4:2:0 use
>1/4th the chroma samples.

Also, MPEG encoding is based upon the key-frame concept. A key frame is
fully encoded. The next frames until it is time for a new key frame,
only encodes the difference. A really smart encoder would be able to
determine that the difference is so great that it should abandon the
last key frame and start a new key frame. I don't know if they exist
or how they do it. 

The easiest way would be to encode it twice and compare the output size. This
would really slow down compression but signifcantly reduce output size. It may
be an option on some encoders. It would not be worth it, IMHO for on the
fly video recording, but it would be worth it for professional production.

In simple terms, a MPEG encoded file of a  camera pointed at a test pattern 
would consist of

	(full frame) 
	(almost no data, mostly headers)
	(almost no data, mostly headers)
	......
	(almost no data, mostly headers)
	(full frame) 

Once the data is encoded, compression is applied. MPEG-1 was designed with
a low enough bit rate that it could be decompressed on the fly and then
DES decrypted in what seemed like real time circa 1985. 

What is commonly called MP-3 audio encoding was one of the specified audio
encoding methods that were part of the MPEG-1 standard. 

MPEG-2 encoding did not include additional audio encoding standards. 

MPEG-4 included an advanced audio codec (usually called AAC, not MP4 audio)
which is able to produce the same quality as an MP3 stream of twice the
bitrate. 

Outside of video encoding, it has been used by the iTunes store.

The world moves in on strange ways, pocket MP4 players were (and in many
cases still are) ones that play low resolution, low compression video files
which are NOT MP4 encoded. The name was picked to make people think they
were better than MP3 players. :-)

Geoff.

-- 
Geoffrey S. Mendelson, Jerusalem, Israel gsm at mendelson.com  N3OWJ/4X1GM