Re: Lectrosonics Quadra Digital IEM Review
Langston, thank you for this detailed and informative review. As I mentioned to you by email, we have been looking at the latency question when feeding the transmitter with digital sources. It wasn't until today that we had the time to set up the test, after gathering the necessary bits.
First, let me point out what we think is a simple error in your math. I've eliminated the 96 kHz specs for simplicity and show only the 48 kHz figures.
You state: "ADC1 + DAC1 48kHz Spec: 3.10ms, Measured: 3.17ms", OK - makes sense. Then here: "Inserting the ADC1 and Quadra in the measurement loop produces latencies of 4.69ms at 48kHz. If Benchmark's latency specs for the ADC1 are close to reality, which they are as shown earlier, then the approx. actual latency of the Quadra system from digital input to earphone output on the beltpack is 3.60ms at 48kHz"
We think what you meant was that if you subtract the ADC1+DAC1 latency of 3.17 ms from the total latency of ADC1+Quadra latency of 4.69 ms, you will reveal the latency of the Quadra wireless transport itself, which would thus be 1.52 ms by your measurements.
We were able to corroborate a similar figure on the bench: we used an A/D converter known to have 3 ms of latency, so: A/D + Quadra = 4.25 ms, less the A/D latency of 3 ms = 1.25 ms.
Interestingly, we tried the analog input to output of the Quadra and came up with a hair under 1.0 ms. (?!?)
Presently, we do not know why this is the case - it clearly should not be, since by eliminating the A/D conversion, we should see a reduction of approximately 0.4 to 0.5 ms. Indeed, this is the difference we see when we compare the analog in to out vs. the digital in to analog out of the D4 system: there is a reduction of .4 ms in bypassing the A/D conversion process.
We are not yet certain if this is a hardware or software bug, but the investigations begin.
Thank you again for the review - sometimes this is how we find bugs in our products. Somewhere between the proto stage and the final product, we perhaps made a mistake of some sort. We'll get to the bottom of it!
