In the recording released Monday, the police officers responding to the report of a break-in at Henry Louis Gates’s Cambridge, Mass. home can be heard switching over to a "channel 2" during the incident.
As someone who spent or misspent much of his youth reporting on Harvard and Cambridge cops, when my friend Marc Ambinder was just a piker, I can affirm that this post basically explains what that’s about. When some kind of an extended incident is underway, officers will go to a back-up channel to discuss it. Not mentioned explicitly is that such a channel is also used for conversations officers may not want broadcast over all the police radios in the city. Usually another channel is used by detectives or drug units. That channel can be encrypted. With some new digital systems, all the channels are effectively encrypted and they may not correspond to specific frequencies, since the new systems can assignt those on the fly
A bit of searching turned up this web guide listing 15 or so Cambridge Police channels. The second channel is described as used for "information."
Not to feed a new birther-style conspiracy, but I have to say I’m dubious about the suggestion that the backup channels are not recorded. Anything’s possible, but even in the old analog days police departments recorded this stuff on multi-track tapes. I think I even requested some one time and got a tape in which several channels were recorded at once, sometimes overlapping.
Maybe with the new, digital radio systems less is recorded than in the old days, but that seems counter-intuitive. I see the main channel is, according to the web guide, simulcast on an old frequency. That could allow scanner buffs, auxiliary cops and off-duty officers to monitor the main channel even without a fancy, official radio. I suppose it’s possible Cambridge only records that old backup of their primary channel, but if so that’s taking a big risk. One of the things most likely to be memorialized on one of these channels is a crime against an officer, so it would seem to me you’d want to keep anything that could provide clues about what happened.
One caveat: police radios in some cities used to have a setting sometimes called "direct" which took them off of the city-wide repeater system and could be used to have a relatively private discussion. But based on what we know at the moment, I don’t think that’s what was going on.