I want to have as long (minutes of playing time) an mp3 as will fit on a standard 700MB CD-R. The rub is that I want this mp3 to be silent - no audio information at all. I just want it to play silence for as long as possible.

I'm assuming I'll be able to make an arbitrarily long silent .wav file as a source.

This leaves two questions, how do I compress this .wav file for maximum data density (longest play time)? Secondly once I know how to encode the .wav file, how long must it be pre-encoding to fit (within a few MB) on an 700 MB CD?

An odd request, but here's the why of it:

The 2005 Subaru Legacy/Outback has a nastily integrated stock stereo with no AUX input. People have figured out how to hardware hack line level into the CD audio path. This requires playing a silent CD while really playing your Aux source. My Outback has the stereo with MP3 capability, so I want to have as long a CD as possible. The non-MP3 head units mute briefly every 72 minutes as the CD mechanism repeats. I want to maximize the time between these brief mutes by using a hyper long silent MP3 track.

Thoughts?

-Zeke

You'd get that by using the lowest possible bitrate in mono. For MP3, that'd be 32kbps. You're interested in the amount of time that would fit in 700MB. That's 179,200 seconds or 2986 minutes and 40 seconds, or 49 hours, 46 minutes, and 40 seconds.

You probably can't actually fit 700MB on a CD, so I'd chop it down a little to make sure you can fit it all there. Even if you call it 24 hours, you'd probably be fine, unless you're a trucker and have a lot of crank.

Just make sure to tell your encoder to do 32kbps CBR in mono. I doubt that your stereo will have a problem with the 32kbps, but it might have a problem with mono. If it can't do mono for some reason, tell your encoder to do joint stereo. You'll probably get nearly as much room as mono. Again, 24 hours should fit and be more than enough.

Very odd hack... Hmm.

I guess the obvious thing is you want the bitrate of the MP3 to be very low. LAME seems to only want to go as low as 32 kbps with MP3, but will go lower (down to 8) with MPEG-2 and MPEG-2.5 . Assuming your MP3CD player is happy with those formats, you might try one of them.

I wouldn't bother with WAV files. LAME can take raw PCM data as input, so I'd just create a file full of all zeroes (use dd on linux, or whatever on Windows) and see how LAME does with that. I don't know for sure that encoding complete silence is lossless (i.e. it won't introduce any artifacts) but if it did, I'd be surprised.

So, now to your question of how long the input file should be... Well, that's trial and error. To lessen the size of the file you'll have to create, you can probably give LAME a sampling frequency argument, -s, which I think only goes down to 8 Khz.

Experimenting with this a little, I have a 100MB file of all zeroes here, and encoding it with this command line, I get a 53 minute MP3 that's only about 12MB in size:

lame -b 32 -s 8 foo.pcm foo.mp3

So, I guess you could fit about 50 of those on a CD-R, so that's around 44 hours... Seems long enough...

Well, I was bored after lunch and before a 2 hour meeting today, so here ya go. Attached is a 12k tar.bz2 archive which will expand to a 610 MB silent MP3 file that runs for 44 hours. It was a fun experiment in compression, to say the least... 4.8 GB all-zeroes PCM file compresses to a 610MB MP3 compresses to a 12k bzip2 archive. Fun stuff!

Anyway, give it a shot...

I love it. You can post a file to the BBS that decompresses to fill an entire CD.

I'd be interested in knowing if that stock stereo will even play the file.

Tony F - Yeah, the reason I have to use a blank CD is that it's a CD changer, and I don't really want to break the ability to play CD's. I figure I'll try a really big .mp3 and see if it chokes. I know I can't have >999 tracks in any given folder, but it said nothing about file length. We'll see.

Quote:
unless you're a trucker and have a lot of crank.

ROFMLAO!

Tony C - Encoding now...thanks for the command line text & zero file idea, much quicker than I would have managed. I managed to max out my RAM using UltraEdit (2GB, never enough...). Encoding will take a while for my 2.1GB .pcm file.

No way! Thanks Tony! I'll be cancelling my LAME encode job now (running slowly for some reason). I'll post back with results to see if the stereo choked.

It works perfectly. It even says 'Empeg' on the readout (it displays .mp3 file name - no tag info even). Perfect silence. 44 hours, 26 minutes of silence.

-Zeke

If silence is golden, that is a lot of coin!

I would be curious to see if there were any compression artifacts. It should have been an easy compression, but that would be an interesting way to test it.

Cool!