Syntrillium Backup Forums :: View topic - File format and compression for radio download

aglady · Posted - Mon Jan 27, 2003 7:48 am

I am a new CEP user and produce news interviews for a government agency in my state. I was able to upgrade from analog, but because of budget woes had to teach myself digital audio with no training. So I have come a long way, baby. But have a lot further to go.

My question is not -- Can you take out the vocals -- although I've already got that too -- but how to reduce my file sizes for faster downloads by my partner radio stations?

I currently record a 3 minute voice interview in .wav, compress to .mp3 and then zip with win.zip before uploading. Because of costs, I am very limited as to the server space my agency currently allows me. Also, some of my radio station partners are in the boonies and have only a connection that is as efficient as two cans strung with string. A three minute cut may take 1 hour to download.

My biggest question is what sampling and bit rates are necessary for there to be high enough quality for radio? Is there a minimum sampling rate? Is there a difference for stereo v. mono? Is there a difference for FM v. AM? What do stations require? Is there such a thing as too much quality – when file size is an issue?

Thanks in advance. I have learned a lot reading the BBs.

Wildduck · Posted - Mon Jan 27, 2003 2:47 pm

OK, I'll jump in. Having read this through and realised how confusing it looks, make sure that you understand the difference between sampling rate of a file and the compressed bit rate.

It very much depends on what the receiving site can cope with and how they will play your contributions.

With .mp3 coding, 16 bit mono files with a sampling rate of 32k compress quite satisfactorily down to 64kbits/sec or even down to 56k. These are speech files of news interviews.

At these bit rates they can be sent over direct ISDN or even modem links in more or less real time. A sampling rate of 32k samples/sec gives an audio bandwidth of 16kHz. FM transmitters normally filter out frequencies above this, so audio at this sampling rate ought to be 'good enough'. This gives a 10:1 compression ratio. A 1 minute uncompressed mono file is approx 4 megs in length at this sampling rate. After compression it becomes 400kBytes.

Stereo files, being twice the size, need to be at least 128kbits/sec when compressed.

If the stations want you to use .mp2 encoding (and you can locate the appropriate plug-ins), it is usually said that a higher data rate is needed.

If you and the stations have broadband connections, straight internet file attachment transfer or posting them on your website might be the way to go.

Always remember that the person sending the file is sending off their 'baby'. But to the person who has to receive it, it is probably just a load more hassle and work. Make it easy at the receiving end.

Hope this helps.