Total posts : 45366
Enabling NRSC pre-emphasis by any means available is pretty much a requirement to match the de-emphsis that is supposed to be in place in the consumer radios. Depending on whether the peak limiter is before or after the pre-emphasis is important to know. In the AMT5000, the internal pre-emphasis is located after the peak limiter. This means when you enable pre-emphasis, the limiter (MODULATION control) must be adjusted downward somewhat to ensure the high-frequency peaks (after pre-emphasis) don’t exceed 100% modulation (by much anyway) to prevent over modulation distortion on the boosted high frequencies.
The Q of the AMT5000 output is such that the 3 dB audio bandwidth is about 11 kHz assuming a “typical” antenna ground resistance of 30 ohms. A lower ground resistance will increase the Q and a higher ground resistance will decrease the Q.
The pre-emphasized high audio frequencies are filtered out by the output Q, so there is no direct need to reduce the bandwidth of the audio to match the RF output bandwidth, but it is important to not over modulate the transmitter at high audio frequencies. High frequency over modulation is not easily perceptible. It usually manifests itself as distortion of the “S” sounds.
It may seem counter productive to have to reduce the modulation level of lower frequencies to achieve the NRSC boost curve. But, that’s the price paid for good high frequency fidelity. When a broadcast station decides to reduce its audio source bandwidth to say 5 kHz, you can see on Rich’s NRSC curve in post #21 above that the pre-emphasis only needs to be 7 dB instead of 10 dB, so the lower frequencies are brought up 3 dB relative to the 100% modulation limit.