04-03-2014, 10:53 AM
Hi Garry,
Being quite basic, the MiniMod audio drops to half at about 50 Hz and 12.5 kHz, but is almost flat between 120 Hz and 5 kHz, and the upper roll off is probably due to the bandwidth of the antenna tuned circuit.
Ideally there should be some pre-emphasis at about 3 to 4 kHz to compensate for IF bandwidth restrictions, and anything above about 7 kHz is normally well outside the IF bandwidth of a normal domestic vintage radio. But even some cheap MP3 players have useful tone correction that can be selected.
Of course 'proper' AM broadcasters must restrict their transmitted bandwidth to not more than about +/- 4.5 kHz so as not to spread excessively onto adjacent 9 kHz channels, although in practice during daylight you wouldn't have two stations nearer than about three channels away in any given service area. And after talking to the Engineer in Charge at Drioitwich (so that was a long time ago!) she told me that AM music stations are allowed a bit more bandwidth than speech stations. All and they have some pre-emphasis just before the higher modulation frequencies are cut.
Then there's the whole complex area of audio compression to make it sound 'louder' and overcome noise, particularly at the edges of a service area. But the whole point of our pantry transmitters is to have a very good signal with maximum audio bandwidth. Luckily that's quite easy to do...
Regards,
Ian
Being quite basic, the MiniMod audio drops to half at about 50 Hz and 12.5 kHz, but is almost flat between 120 Hz and 5 kHz, and the upper roll off is probably due to the bandwidth of the antenna tuned circuit.
Ideally there should be some pre-emphasis at about 3 to 4 kHz to compensate for IF bandwidth restrictions, and anything above about 7 kHz is normally well outside the IF bandwidth of a normal domestic vintage radio. But even some cheap MP3 players have useful tone correction that can be selected.
Of course 'proper' AM broadcasters must restrict their transmitted bandwidth to not more than about +/- 4.5 kHz so as not to spread excessively onto adjacent 9 kHz channels, although in practice during daylight you wouldn't have two stations nearer than about three channels away in any given service area. And after talking to the Engineer in Charge at Drioitwich (so that was a long time ago!) she told me that AM music stations are allowed a bit more bandwidth than speech stations. All and they have some pre-emphasis just before the higher modulation frequencies are cut.
Then there's the whole complex area of audio compression to make it sound 'louder' and overcome noise, particularly at the edges of a service area. But the whole point of our pantry transmitters is to have a very good signal with maximum audio bandwidth. Luckily that's quite easy to do...
Regards,
Ian
