eDrum Latency Measurement - Page 1
Forum section: electronics / beginners

author: metrologist
author_rank: Super Contributor
timestamp: November 13, 2022, 04:27:59 am
content: I bought the top-of-line of a low-end eDrum set and wanted to measure the latency. This is the time from when the drum stick hits the drum to when sound is produced from the audio amplifier.Here's what I got by placing a small speaker face down on the drum head and striking the magnet. The O-scope yellow trace is connected to that speaker. The blue trace is connected to the audio line output of the eDrum module. I'm seeing 6ms. Is this a valid measurement and what could be the margin of error?I gave it many whacks and get the same timing results. Below a soft and harder strike.Another question is what could be the small bursts on the line output?

author: 3roomlab
author_rank: Frequent Contributor
timestamp: November 13, 2022, 04:50:39 am
content: MIDI / SMPTE / BPM clock noise? (scratching my head, what is 200Hz period? more like BPM? I am not sure)Which drum did you get? Yamaha? Roland?

author: Terry Bites
author_rank: Super Contributor
timestamp: November 15, 2022, 04:03:37 pm
content: The speaker coil and cone will have have significant damping. There is mass and spring inertia to overcome, that will add some delay. A Piezo transducer would probably be faster and have a much sharper impulse response.

author: metrologist
author_rank: Super Contributor
timestamp: November 15, 2022, 04:36:53 pm
content: That's is what's in the drum, some have a couple or a few. I think ideally we would use our own transducer, but some split the signal that comes out of the drum to the control module. I think I have one of those laying around.Something like this:

author: EPAIII
author_rank: Super Contributor
timestamp: November 16, 2022, 09:38:52 am
content: It looks pretty valid to me.Error sources? Well, you are using two completely different ways to detect the original strike and the sound from the speaker. If you want to even that out, get two identical microphones and mount one next to the drum and the other next to the speaker. Set them equal distances away. And have the speaker in another room with the door(s) closed to avoid interference from one to the other. If you want to be real picky, use a microphone cable on the drum's mike that is equal in length to the speaker wire plus the microphone cable on the speaker's mike; but, with 2/3 the speed of light being what it is, I doubt that is really necessary. The errors from cable length will be many orders of magnitude less than what you seem to be measuring.As for the difference in the waveforms, I wouldn't expect a drum to produce a sine wave or even something close to it. The simplicity of the yellow waveform is probably due to the characteristics of the speaker you are using as a sensor. And remember you are striking that speaker, not the drum. And that speaker must transmit that strike to the drum.Come to think of it, it will take some time for the mechanical impulse to travel from the point on the speaker where you strike it to the point/area of the speaker that rests on the drum. This is due to the speed of sound in the material of the speaker, probably steel. And that is a lot slower than 2/3 the speed of light. So no, that is not a good way to do it. Strike the drum and use the two mikes as I described.

author: metrologist
author_rank: Super Contributor
timestamp: November 17, 2022, 07:04:04 pm
content: Someone measured the time it took an impulse to travel through a metal rod. 180 µs.

author: John B
author_rank: Frequent Contributor
timestamp: November 17, 2022, 07:40:11 pm
content: I don't think the designers need to aim for much faster than ~6ms. Even studio gear that advertises low latency is still in the realm of ~3ms.