Characterizing the FT 991A USB audio processing latency when traversing a PC
When operating the FT-991/A with a side-to-side PC, you may notice that there's a slight delay on the PC audio when comparing to the radio speaker output.
The FT-991/A features a TI Burr-Brown PCM2903B USB Stereo audio codec, which is fed from the same audio chain that outputs an analog audio to the RTTY/DATA port. This downstream audio stream roughly translates to: M62364FP DAC, tee output to DATA/RTTY pin 6, PCM2903B, USB hub, USB port, USB cable, PC USB port, PC USB hub, data processing by the PC, data output to the PC sound card, audio output to the PC speakers. That surely adds delay to the PC audio. But, how much?
For the reception (RX) test, I used a Rigol DS-1054Z hooked to the FT-991A front Phones jack to Channel 1, while I connected PC speaker port to Channel 2. The radio was in FM mode, and I broke squelch by pushing the SQL OFF function. The oscilloscope was triggered to Channel 1 rising edge, and used the Oscilloscope Cursor function to calculate the delay between the beginning of the AF output in both channels.
During the Transmit (TX) test, the PC was connected to the Channel 1, while the FT-991A Phones jack were hooked to the Channel 2. I used the Data-USB mode, and transmitted a 1 kHZ tone. In order to assess the delay, I used the Monitor functionality, which would output the actual transmission audio on the FT-991A Phone jack.
Device Under Test
The PC is a Lenovo W541, running Windows 7. Two tests were conducted for the reception test: the first one, using the PC's standard audio processing chain, by bridging the radio's AF output to the PC speaker, and a second one using VoiceMeeter Banana mixing and controlling the audio output.
A single test case (only using VoiceMeeter) was performed during the Transmission test.
The raw Windows audio processing delay was 104 milliseconds:
FT-991A Raw Windows 7 Audio Processing Delay Time
Interestingly, when using VoiceMeeter Banana, I got a slight (~ -10%) improvement on audio processing latency, sporting 88 milliseconds of total latency:
FT-991A Windows 7 VoiceMeeter Audo Processing Delay Time
The only scenario tested was using VoiceMeeter Banana as the audio processing tool. The result was nonetheless... Impressing! The delay between the PC transmit and the FT-991A output was a pale 20 milliseconds.
PC and FT-991A AF out Transmit latency
The delay is slightly noticeable by human ear, and less than noteworthy for current digital modes during the reception. However, the transmission latency impressed, even after traversing the entire processing chain.
This is the heavily redacted FT-991A AF out block diagram.
FT-991A AF Out Block Diagram
Hope that helps - 73s de PY2RAF.
