[mythtv-users] Recording Bitrates fpr PVR 350

Michael T. Dean mtdean at thirdcontact.com
Fri Dec 23 12:09:08 EST 2005

Philip Shead wrote:

> Michael T. Dean wrote:
>> Scott Alfter wrote:
>>> Disk space is cheap.  I just record everything at 6 Mbps and call it 
>>> a day.
>>> With ~340 GB (real gigabytes, not "salesman's gigabytes") across 
>>> three drives,
>>> I've never run out of space. 
>> 1 gigabyte = 1GB = 1,000,000,000 bytes
>> 1 gibibyte = 1GiB = 1,073,741,824 bytes 
>> (gibibyte as in "gigabinary") 
>> So, a "salesman's gigabyte" is a "real gigabyte."  I think you're 
>> trying to say you have ~340 "real gibibytes." 
>   Too bloody right. Now if you'll excuse me one of my mythboxs
> memory modules failed and I've found myth can be a struggle
> with less than 536MB. Unfortunately nobody locally seems to
> stock 268MB memory modules ?!


> (Back to myth.. are bitrates in the configuration setion in
> mythtv in metric kilobytes or binary ones?)

All _should_ be "metric" in video/audio applications, just like with 
network bandwidth.  (Yes, 100Mbps is 100,000,000 bits/sec.)  (See also 
http://en.wikipedia.org/wiki/Bit_rate (specifically under "Usage 
Notes").)  However, in some cases, the values may be passed through to 
other software (i.e. hardware encoder drivers, which themselves may pass 
the value to firmware), so ...

I have to admit, though, that I didn't dig through any source code to 
verify Myth's/ffmpeg's usage and that Google won't help with figuring it 
out (because people post information using what they think it means).  
However, since Myth is using ffmpeg--which (like Myth itself) is written 
by people who know A/V--I'm guessing it should be pretty standard for 
bitrate in most contexts.  I would guess, though, that most of the 
storage measurements are "traditional" binary-interpretations of the 

Wikipedia suggests that when in doubt, bit measurements should be taken 
to use "decimal" interpretations and /only/ byte measurements should 
ever be interpreted to use "binary" interpretations.  ( 
http://en.wikipedia.org/wiki/Binary_prefix )  As a matter of fact, it 
goes so far as to say, "Certain units are always understood as decimal 
even in computing contexts.  For example,  hertz (Hz), which is used to 
measure clock rates of electronic components, and bit/s, used to measure 
bit rate."  (Yes, this also says that your 3GHz processor runs at 
approximately 3,000,000,000 Hz.)

> Someone who spent months failing to work out why his attempt
> to make nice half dvd size GP mpeg4s was because he was using
> metric not whilst mencoder uses binary. Now I have a nice
> spreadsheet to do it for me.

In the DVD world, a 4.7GB DVD can hold 4,700,000,000 bytes (and I won't 
even get into the fact that a 700MB CD can hold approximately 700MiB of 
data...) and a DVD can hold a stream such that the sum of video, audio, 
and subpicture bitrates has a maximum of 9.8Mbps = 9,800kbps = 
9,800,000bps and the total "mux rate" has a maximum of 10.08Mbps = 
10,080kbps = 10,080,000bps and the information file plus DSI packets 
have a bitrate of 1.0Mbps = 1,000kbps = 1,000,000bps giving a total user 
data rate of 11.08Mbps = 11,080kbps = 11,080,000bps.

Of course, this doesn't help when a programmer says, "we don't need no 
stinkin' standards," and writes an A/V app to use binary interpretations 
of the prefixes.


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