Software, technology, sysadmin war stories, and more. Feed
Monday, December 10, 2012

My roundabout mix tape, CD, Oggs, radio station, and ...

If you're old enough, you should remember the whole "mix tape" thing. I used to make my own compilations of things based on what was played on the radio. I couldn't just copy it from actual albums because that would have involved paying real money. Back then, if it didn't get radio play, it didn't exist as far as I was concerned.

I had a boom box which had a pair of tape decks and could record straight off the radio, so that plus a handful of empty tapes got me going. I used to leave a "work tape" in the record-capable deck while listening to the radio. If something good came on and it wasn't something I already had on tape, I'd have to reach over and start recording as soon as possible. This meant most of my recordings missed the very beginning of songs, but I could live with that.

After getting a good copy of a song, then I'd shuffle things around and append it to my latest collection. This involved putting the collection tape into the record deck and moving the work tape over to the play-only deck. Then I'd have to cue them both up and release pause on both at just the right time to get it going. After that finished, it was time to switch things back and wait for more stuff to come on.

Over time, I built up a nice little snapshot of what was being played on the radio at one point in my life. I brought these tapes with me even after moving to far less interesting places with radio stations to match. I played them and played them, and they probably stretched out just a little more every time I did it.

Then one day, things had finally gotten to where wrangling raw audio was no longer a big deal, and I was able to make one huge WAV file from an entire tape. I took this raw file and played through it while making notes about index points: where tracks should begin and end, and whether a "lead-in" was appropriate or not. This is where your CD player will actually show a negative time and will count *up* to 0:00. I used that for the random gunk which would inevitably show up between actual songs: bits of ads, DJ chatter, and so on.

With this raw audio and the cue list, I burned a couple of CDs and called it done. Now I had all of the music in a nice format which wouldn't degrade any more just because I played it, and it was indexed with random access too. This was the state of affairs for a couple more years.

Things changed some more and it was time to bring that music back online permanently. I ripped the CDs I had once burned myself, and encoded all of the audio as Ogg Vorbis. That was the official non-evil audio format at the time, and I went with it. This went into my personal library and would be played through the computer along with other songs on a random basis.

Even later than that, I started wanting to be able to hear my music even when I wasn't right next to my computer. Fortunately, I had one of those little battery-powered FM transmitters which have a decent range. You plug it into the headphone jack of an audio device, pick a channel, and you should be able to hear your tunes on a nearby radio. I started that running, and then proceeded to tune in my own local FM station... on the same boom-box radio which recorded the music years before.

Tracing the music reveals the following path. First, it was stored on some unknown medium (probably a CD) at the radio station. Then it probably went out through a studio to transmitter microwave link from their offices to the transmitter site outside of town. From there it rode back into town over their better-known commercial FM broadcast frequency and into my radio. My radio handed it off internally to the tape deck, and it was laid down as a bunch of magnetic patterns.

It stayed like this, then came out of another tape deck (one with a line-out jack, since I cared about levels) and went into my computer's line-in, was digitized by a sound card, passed through a bunch of code, and wound up in a WAV file. That file eventually was handed over to the CD-R drive and commanded the laser to create the right pattern of pits and lands on the media.

After that, those pits and lands were detected by the playback laser and were turned into another series of WAV files. Those files were read, analyzed, and the audio content was turned into oggs. Then those files sat around until it was time to play them. They shuffled off the disk, through Apache, out through the network interface and across a wireless gateway into my laptop where they were decoded into audio data once again. That then emerged from the sound card where it was turned back into analog audio on a 1/8" jack and picked up by the transmitter.

The transmitter took its base frequency I had selected and modulated it appropriately to convey the audio it had been given. This then went over the air on a low power signal back into the same boom box from years before, where it was turned into a raw audio signal once again, and was then routed to the speakers. They pushed some air around and I got to hear my music.

That's the nuts and bolts of it, and I'm sure I missed a bunch. Alternatively, I could just say that I recorded a bunch of tunes a long time ago and kept them around so I could listen to them years later. It's all a matter of who you're talking to and whether they care about the nerdy technical details or not.