The REA AMM-SD1 AM modulation monitor is quite nice! It consists of two boxes plus Windows software. One of the boxes is an envelope detector that outputs DC plus audio. The second box does an analog to digital conversion of the envelope detector output and sends it over USB to the display software. The ADC and USB interface are handled by a PIC24 microcontroller. It seems that the USB could just carry digitized audio from an ADC that does not have an input HPF so the DC and audio are passed to the Windows application. However, I have not tried to trace the circuitry or analyze the software. It just works!
The video at the right shows the monitor in operation. The envelope detector is in the RF line after the Dentron Clipperton L amplifier and before the DC bias Tee that puts DC on the coax to drive the MFJ 998RT antenna tuner. The amplifier is being driven by an E F Johnson Viking Ranger transmitter. The video shows the monitor working with voice and with tone (holding a microphone calibrator up to the microphone).
Video showing the AMM-SD1 in operation. Note the negative peaks are higher than the positive peaks indicating weak tubes in my Dentron Clipperton L amplifier. The image in the background is the waterfall graph from the KFS Web SDR.
Inside the envelope detector module. The output is a DC voltage containing the carrier level plus AC representing the modulation.
The USB interface. This module accepts the output of the envelope detector and sends it as a stream over USB for interpretation by the Windows software. This module uses a PIC24 microcontroller.
For comparison, there's a photograph of a General Radio 1931A modulation monitor.