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Re: AM radio recorder

dbp lists writes:
>I've been thinking for a while about setting up my debian box to
>record some AM radio programs... sort of a Tivo for radio.  A lot of
>mp3 players come with FM radio and recording function, but not AM


>So I've been thinking about just using a radio (with AM capability,
>obviously) and using an 8mm plug (male conneciton on both ends) and
>connecting the output of the radio to the input (microphone socket) of
>my soundblaster sound card on my linux box.   THen I can use some bash
>scripts and "at" commands or cronjobs to record the shows I want.

	You're almost there.

	First, be sure the alsa drivers are built in to your kernel
and that they are for _YOUR_ particular sound card.  Being a
SoundBlaster makes life easier for you since there is lots of support
for SB's.

	Second, the Microphone jack is a 3-conductor stereo-type
miniature phone plug that is actually used in a somewhat special manner.

	The tip of the plug is the audio input, all right, but the
first ring is a low-level DC power source from your sound card whose
purpose is to supply DC to the FET-based preamp in most computer
microphones.  That gets the impedance down to a lower level for the
audio output and amplifies the condenser-type mike element that is in
most computer microphones.

	Also, microphone inputs only need a few millivolts to work,
like 3 or 4, and your radio probably puts out that much audio with the
volume turned all the way down.  If it is a tuner, it will probably
be putting out up to 2 or 3 volts which is about 500 to 1,000 times
more audio than your sound card's microphone input is expecting and
you'll discover that you can't turn it down enough to get good sound
without distortion.

	You can either try to procure or build an attenuator to knock
off about 50 DB of output from your radio or you can feed the sound in
to the Line inputs on your sound card.  That doesn't even require
anything but the patch cord.

	If you get alsa working, your /dev/dsp standard device will
work as will amixer and other alsa-compatible programs.

	A couple of final thoughts.  The sleeve or ground connection
on your audio card may be common to the case or frame of your
computer.  This has some safety implications when connecting it up to
external devices plus some related sound-quality issues.  Sometimes,
there will be a little bit of AC power current flow between the Earth
ground of your computer and the audio source you are feeding to the
sound card.  If there is a lot of flow, it  can actually be dangerous
to you and to your equipment.  Usually, however, the effect is to put
a hum on the audio that you can't get rid of to save your soul.  The
fix is to do what the pros do and buy little transformers called audio
isolation transformers to go between your sound card and the audio

	If you get a 3-conductor mail to mail patch cord, that would
patch a stereo headphone output to the two input channels of your Line
inputs.  If you can get the level right and there is no hum, you are
home free.  There's still no way to tune the radio from your computer,
but you certainly can use at and cron jobs to start and stop your
recording on a schedule.  Read the HOWTO's about Linux audio for more
good information.

Martin McCormick WB5AGZ  Stillwater, OK 
OSU Information Technology Division Network Operations Group

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