FM Radio question

[space_cowboy]

How do stations mix and broadcast the signal? I have a local station, 100.30 "JACK FM" that pretty much destroys every other input I have. Really, I have never heard these mid horns sing until now. If you think La Scalas don't have bass..........tune in. Really, this is blowing me away.

Anyone else have luck with FM? My FM stage is nothing fancy, but maybe I got lucky somehow. Anyhow, comment on your FM experiences.

Mark

[dougdrake]

Funny. We have a Jack FM here in Denver, too, on 105.5. Their ads make them look local, but aside from college radio (thank God for that - KBCO in Boulder), I guess no radio's local anymore.

[79 Scottsdale]

test

[bdc]

I think there is a really old thread about this, somewhere.

[79 Scottsdale]

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On 7/24/2004 9:08:36 PM space_cowboy wrote:

How do stations mix and broadcast the signal?

Mark

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Are you sure you want the answer to that?!

To transmit modern MPX stereo, the following stages are needed: pre-amp left, pre-amp right, summing amplifier A, summing amplifier B, inverter, delay, balanced modulator, doubler, 19KHz pilot oscillator, final amplifier, and a transmitter.

Two signals are needed to accomplish FM MPX transmission: L+R and L-R. The transmitter output is a linear mix of the L+R signal.

To form the L-R signal, the left channel is applied directly to summing amplifier B. However, the right channel passes through an inverter to become the -R signal before being applied to the summing amplifier. The output of summing amplifier B is a linear mix of the left channel and an inverted right channel, or the L-R signal.

The L-R signal is then applied to a balanced modulator. A 38KHz carrier is also applied. This carrier is derived from a 19KHz master oscillator using a frequency doubler. The output of the modulator is the double sideband, supressed carrier L-R signal. This signal is then coupled to the FM transmitter through an amplifier. It is joined by the L+R signal which was passed through a delay network to compensate for the balanced modulator delay of the L-R signal. The 19KHz pilot is also applied to the transmitter.

In FM broadcasting, the maximum allowable carrier deviation is +/- 75KHz, this is true for both monaural or MPX broadcasts. Therefore, when the modulating signal is MPX stereo, deviation by the broadcast is slightly reduced. This is a result of the 19 KHz pilot carrier, which is always present, but does to change in amplitude. It deviates the carrier 10% of its maximum deviation, or +/- 75KHz x 10%, which is equal to +/- 7.5 KHz. Thus, the maximum deviation of the audio signals is 90% of 75KHz, or 67.5 KHz. The difference between 90% and 100% deviation is minimal and not readily discernible to the listener. It may also seem that this remaining 90% deviation must be divided equally between the L+R and L-R signals. However, in practice, a process called interleaving takes place. When the L+R signal is at its peak amplitude, the L-R signal is at minimum amplitude, and vice versa. Therefore, due to interleaving, each modulating signal can independently produce almost full deviation without over deviating the carrier.

Best Regards,

Ryan

[ben.]

What you're hearing is the compression and EQ applied to nearly every FM broadcast signal. I can sound quite good when not overdone. Compression is of the dynamic range sort, not data compression to reduce digital file size. Done right, it can offer a nice, full sound; coupled with EQ you get that big, impressive sound you're describing. While fun, I find it obscures details and subleties that I look for when listening critically. Done wrong, it can just sound like crap, too. Wehn I DJed in college at a good size club (for Fredonia, that is...), I ran a stereo compressor and EQed the bejeesus out that ratty rig to get it to sound halfway right.