[BC] Pre 1960 radios...

[Kevin Tekel]

Harold Hallikainen
> Was it really necessary to drop the audio response down to 3 or 4kHz to
> get rid of a 10kHz whistle? Didn't radios in the 1930s have a 10kHz notch
> filter (possibly a twin-T)? Some radios had selectable bandwidth. In a
> car radio, the bandwidth select could possibly be tied to the instrument
> lights, which were already fed to the radio to light the dial.

It really wasn't an issue until the advent of "hi-fi" systems in the 1950s.
The band wasn't that crowded yet and speakers and audio amps didn't have
that much high-end response at 10 kHz, regardless of the tuner's bandwidth.
Only a few top-of-the-line radios in the '30s and '40s had whistle filters.

By the late '50s when experimental stereo broadcasts were done with one
channel on AM and the other on FM, a range of "hi-fi" AM tuners were
introduced, with improved audio bandwidth and 10 kHz notch filters.  Some
even added *pre*-emphasis to the output of the AM tuner, to compensate for
the treble roll-off of stations still using old low-fidelity equipment.

Some shortwave radios also had a selectable 5 kHz notch or low-pass filter
to help reduce carrier whistles and adjacent-channel noise in crowded band
conditions.  Other radios, including car radios, had a selectable bandwidth
disguised as a "Bass/Treble" switch.  "Bass" is narrowband and "Treble" is
wideband.