doctor who - How come the old audio tracks are mostly preserved while the film is not? - Movies & TV

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I've been watching the
First Doctor serials with William Hartnell, and many of the episodes had to be
reconstructed because much of the original material was lost (apparently by the BBC
cleaning up). However, I wonder; while the video feed has to be stitched together from
stills some times, the audio track seems to be consistently preserved (well, not always
in the best quality, but there is an audio track as far as I've watched, which is about
20 serials).

Why is that? What is the background
of the lost episodes? Which circumstances led to the video track being lost while the
audio track was preserved?

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class="normal">Answer

href="http://www.recons.com/aboutrecons.htm">This site talks about how the
reconstructions were made. Here's what it says about why there are missing episodes at
all:

...By the
1970's the show had moved to colour and the old 60's black and white episodes were
becoming less popular to show as more and more foreign TV stations opted to show the
newer colour stories instead. The copies distributed abroad were supposed to be either
destroyed or returned to the BBC after use. Believing that the BBC film
archive held copies of all Doctor Who stories, BBC Enterprises decided that their old
black and white film prints would no longer be required for overseas sales and their
destruction began in the early 1970s.

It was later discovered that the film
Archive did not keep copies, all the original prints and negatives had been passed to
Enterprises. By the late 1970s virtually all the 60s episodes had been
destroyed. Copies of many episodes have subsequently been found in other BBC archives
and returned from foreign TV stations and private collectors.
...

...and here's
what it says about why we have audio from all of
them:

Although the
BBC kept some of the sound effects and music used from the 60s episodes, the full
soundtracks were not recorded separately. However, several fans did record
the stories on reel-to-reel audio tape from their televisions upon their original
transmission, most notably David Holman, Richard Landen, David Butler and Graham
Strong. Most of the audio recordings that were taken had the opening and
closing credits cut out at the time of recording to save on tape, so although all the
episodes do exist on audio, there are a few seconds at the start or end of some episodes
that are missing. For this reason it is often necessary to combine several different
versions to produce a complete soundtrack and the exact start/finish points of episodes
are often difficult to
determine.

Presumably
this also helps explain the quality issues.