AV News 196 - May 2014
Returning to Dyl's question - suppose the filter on the mic has a turnover
frequency of 150Hz and that on the recorder is 80Hz, you have the option, in
increasing order of severity of:
both filters off
recorder filter on, mic filter off
mic filter on, recorder filter off
both filters on.
There may well be other considerations. If your mic is totally enclosed in a
windshield with a furry cover, it is quite a palaver to get at the mic filter switch so,
other things being more or less equal, you will switch in the recorder filter if you
need it in a hurry
There is one more consideration, which only affects certain combinations of
equipment, and which I have only just encountered myself (after 50 years'
recording!) Before describing it we must remind ourselves that the wind 'rumble'
that we hear is just the tip of the iceberg. The overwhelming majority of wind
disturbance will be infrasonic - at too low a frequency for us to hear (and very
likely too low a frequency for our speakers to reproduce.) However, whether we
hear it or not, the signal is still present, and can cause some quite obscure
problems - the main ones, and how to avoid them, are demonstrated on 'All About
Microphones.'
It was very windy and I needed to filter. For speed I used the recorder filter.
All seemed OK on the recording level meter, but when I played back the
recording at home it was awful. A few milliseconds of signal was missing every
few tens of milliseconds. This gave an extremely 'rough' character to the sound.
I soon realised what was happening. The mic in question (Sennheiser K6 series)
has a very high output and a good low frequency response, so the wanted audio
signal was riding on an enormous infrasonic wind rumble signal. This infrasonic
signal was overloading the mic input stage, which is before the mic input filter. At
the peak of each overload, the wanted signal was 'out of range' of the electronics,
so was briefly silenced. Downstream of all this, the mic input filter removed the
infrasonic signal leaving the sort-of-intermittent audio. This looked OK on the
meter, but sounded awful.
The solution was simple - use the filter on the mic so no large infrasonic signal
was presented to the recorder. That worked brilliantly.
'All About Microphones' is one of a series of tutorial CD-ROMs aimed at
answering all the FAQs and avoiding all the FHMs (frequently heard mistakes.)
They can save you both a lot of frustration and, by helping you buy the right
equipment first time, save you money too.
1/4" Tape Reels
Here in the Channel Islands, there are not many occasions to link up with
other members. I have a quantity of 1/4" tape reels, Scotch 207 3600ft,
which are mostly used once only and may be of use to someone with a reel
to reel recorder at low cost.
Contact: Eric Ferbrache ARPS
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