[mythtv] links and features

Michael T. Dean mtdean at thirdcontact.com
Tue Jun 29 15:43:43 EDT 2004

Marcos Pinto wrote:

>this looks like a great app so far from what i've seen.

>here are a couple of questions that i hope someone out there can answer for me.  
>for high quality recording, what's roughly the hours per MB ratio?
Depends on the CODEC you're using.  It would be about 0.000488 for 
MPEG-2 and about 0.000244 for MPEG-4.

Answering the question I think you meant to ask ("How much storage is 
used per hour of high quality video?") is actually quite 
difficult--primarily because of that little phrase, "high quality."  It 
seems that "high quality" is a matter of opinion (or, maybe, religion).  
Therefore, the following discussion is primarily meant to give you a 
good "rule of thumb."

Storage requirements are completely dependant upon the bitrate you're 
using to record the data.  However, the bitrate should be adjusted 
depending on the resolution at which you're capturing to provide 
efficient storage without sacrificing quality.  Also, it's possible to 
use variable-bitrate (VBR) encoding--which would mean that the actual 
bitrate would fall within some range (between the minimum and maximum 
you specify) but would constantly change during recording depending upon 
the actal images being encoded.

However, I think most would agree you can get a good picture using about 
2GB/hour for MPEG-2 and about 1GB/hour for MPEG-4.  For example,

Let's assume "DVD-quality" recording in NTSC (US standard) format to 
MPEG-2 (i.e. using a hardware based encoder like the PVR-x50 cards).  
NTSC uses 720x480 resolution with a video bitrate between 3000 and 8000 
kbps and an audio bitrate between 192 and 448 kbps (PAL/SECAM is 
identical except the resolution is 720x576--therefore, given the same 
bitrate, it would have identical storage requirements, but because of 
the higher resolution, it may warrant using a higher bitrate).  
Therefore, assuming a constant video bitrate of 4500kbps and a constant 
audio bitrate of 192kbps:

4500kbps + 192kbps = 4692kbps / 8bytes/bit = 586.5kBps * 1024B/kB = 
600576 B/sec * 3600 sec/hr = 2162073600 B/hr / 1024 B/kB / 1024 kB/MB / 
1024 MB/GB = 2.01 GB/hr

Television broadcasts, however, are not generally high-enough quality 
(assuming standard definition TV) to warrant DVD-like resolutions.  You 
may be able to reduce resolution (to 640x480 or 480x480, for example) 
and, therefore, bitrate while maintaining quality approximately 
equivalent to the source material.  Storage requirements will be change 
by 0.215GB/hr (219.7MB/hr) for every 500kbps change in bitrate.  
Therefore, if you use 5000kbps for video bitrate, you would need 2.23GB/hr.

Generally, MPEG-4 recordings will take approximately half the storage of 
MPEG-2, for nearly equivalent quality.  Therefore, IMHO, a good rule of 
thumb would be 2GB/hr for MPEG-2 and 1GB/hr for MPEG-4.  The docs ( 
http://www.mythtv.org/docs/mythtv-HOWTO-3.html#ss3.1 - under Hard 
Disk(s) ) say to expect a range from 700MB/hr to 2GB/hr for MPEG-4 and 
larger for MPEG-2 and RTJPEG.  You may be happier making guesses based 
on the higher storage requirements if "quality" is very important to 
you, but I recommend trying the lower bitrates to see if you can really 
tell a difference and whether that difference is worth the added cost.

>in mythdvd, is there any work being done to allow users to not only copy
>the dvd to the hd, but to also(instead) make a backup to another dvd?  
>in mythmusic, is there any work being done to allow the user to burn a
>music cd?
Don't know.



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