Saturday, January 25, 2003

MPEG1 Settings

Just a note for future capping. With the ATI digital VCR, recording MPEG1 at 5Mbps, 640x480, Motion Estimate Quality of 25, and 44kHz 16bit Stereo audio results in a file that is 635Kb/sec (or 1.09Gb per 30 minutes). I think the audio portion is included in the 5Mbps value, since that calculates out as 640Kb/sec (of which 172Kb/sec was the CD quality audio-stream). It might be worth it to record at 22Khz instead of 44Khz for when I'm pulling these old clips off of older VHS tapes, freeing up 86Kb/sec for video data. (Especially since I'm going to convert to 22kHz audio during the compression phase anyway.) Audio sync is clean as well (I've had issues in the past with this too).

On the output, I'm currenting storing to the DivX5 codec, using pretty standard settings, bit rates of 500-1000Kbps. As I mentioned I'm converting the audio to stereo, 22kHz, 16bit - but compressing it with Microsoft's ADPCM 22kHz, stereo, 4bit format (this helps with the audio sync error that can occur from using something like MP3 audio for the audio track). I forget whether the DivX5 encoder's bitrate setting includes space for the audio information or if that gets tacked on the top of the setting number. Anyway, here's my current short list of bit rates and storage needs:

5Mbps (640Kb/sec) - 1hr (2.14Gb) - 19min on a 700Mb CD - 2hrs on a 4.7Gb DVD - used for decent-good quality video sources when recording with MPEG1

4Mbps (512Kb/sec) - 1hr (1.71Gb) - 24min on a 700Mb CD - 2.5hrs on a 4.7Gb DVD - I'm not using bitrate this at the moment, opting for the 5Mpbs instead, but I'd expect the quality to be close to the 5Mpbs, maybe good for only low-decent quality source video using MPEG1.

1Mbps (128Kb/sec) - 1hr (0.43Gb) - 95min on a 700Mb CD - 10hrs on a 4.7Gb DVD - I may choose this bitrate to store my better quality clips using DivX5, or I may sneak down to 700-850Kbps instead to get just a bit more space out of a CD (US shows are 24min without commercials, so to get 96min/CD I've got to go down to 950Kbps or a bit lower to fit 4 on a CD).

750Kbps (96Kb/sec) - 1hr (0.32Gb) - 125min on a 700Mb CD (VHS tape capacity) - 12.5hrs on a 4.7Gb DVD - Seems to be a good quality level.

500Kbps (64Kb/sec) - 1hr (0.22Gb) - 190min on a 700Mb CD - 20hrs on a 4.7Gb DVD - This is about as small as I'll go since I'm archiving material, not sizing it for distribution.

Update: I just looked at somethin I encoded earlier. A 1h28m41s clip encoded into DivX5 at 780Kbps with MS ADPCM 22Khz/Stereo/4bit audio ended up at 641.8Mb (860Kbps or 108Kb/sec). Which tells me that the DivX encoder setting is for the video data only, and the audio gets tacked on top of that. So if I want 1Mbps on the output, I have to plug in 920Kbps for the DivX codec and allow 80Kbps for the audio information (which makes me wonder if the MS ADPCM compressed the audio at all).

Also, on an AthlonXP 1800+ system with DDR memory I'm encoding at a rate of about 24-25fps (just a crop filter in VirtualDub). While not quite real-time, it's close enough that I can queue up 4-5 hours of clips at a time for encoding and have them done nice and fast.

posted by Wuphon's at 9:31 PM