Background: Measuring the XDR's FM Frequency Response from Antenna In to Line Out
Data suggests the XDR's analog audio preamp circuit has drooping treble response. What if Sony did that on purpose to compensate for a corresponding treble boost in its digital processing? Seems unlikely, but not impossible.
To test the aggregate frequency response, I used my HP 8903B Audio Analyzer to generate clean audio test tones at selected frequencies. The test tones were fed into a Panasonic FM Signal Generator and used as the modulation source during testing. The Panasonic then generates an FM signal at the selected frequency on the FM band and modulates it with the HP's audio test tone. This is essentially broadcasting precision test tones on 97.3 MHz.
Analog FM frequency response is usually stated as being 30 Hz to 15 KHz so that's the range I measured. (In the graph, the 30 Hz figures are capacitance-related anomalies and should be disregarded: The XDR’s bass response is down only about -0.18dB at 20 Hz with my audio mods.)
Note About FM Pre-Emphasis: FM radio uses "pre-emphasis" at the broadcast side, and a corresponding "de-emphasis" at the receiver side. It's kind of like the RIAA curve used for vinyl records. In North America, the standard is 75 microsecond pre-emphasis. I used reference data found online to calculate the expected deviation from flat response, then compared the expected deviation to the actual measured deviation.
The table & graph show the measured overall frequency response for Unmodified and Modified XDRs, from Antenna In to Line Out, with compensation for FM de-emphasis. In the graph the 0.00 line is the 50 Hz reference, with deviation in +/- dB from there. The treble dropoff on the Unmodified XDR from Antenna In to Line Out matches up pretty closely with the same measurements when confined to the XDR's analog audio preamp circuit.
Disregard the 30 Hz measurements: They're anomalies related to circuit capacitance.
Conclusion from this data: The XDR's treble droop is in its analog audio preamp circuit, not the tuner module or DSP section. It can be corrected!
Improving Audio Quality
A few new parts and adding moderate negative feedback to the stock audio preamp circuit will substantially and measurably improve the tuner's audio performance.
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Sony gave the XDR amazing reception capability, and its DSP section is very high quality. But its analog audio preamp can be improved:
The audio analyzer plots below are for the unmodified XDR audio preamp.