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Author Topic: Tascam DR-70D - modification discussion  (Read 20780 times)

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Offline voltronic

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Re: Tascam DR-70D - modification discussion
« Reply #30 on: July 27, 2015, 11:20:21 AM »
Spectrum analysis added to the dropbox folder.
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Offline voltronic

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Re: Tascam DR-70D - modification discussion
« Reply #31 on: July 27, 2015, 12:53:50 PM »
^ This is why 2manyrocks and I have been discussing ways to equalize conditions to separately perform your "pillow test".  We both own CM3s, he has a stock 70D and mine is modified.  I'm proposing going a step further than the pillow by wrapping each mic in a towel first to avoid HF leakage, and then in fleece blankets or acoustic foam, since we both have some.  It's not perfect, but shirt off someone else doing a before and after themselves, it's the best we can get in terms of what you're suggesting.
« Last Edit: July 27, 2015, 12:57:58 PM by voltronic »
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Offline 2manyrocks

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Re: Tascam DR-70D - modification discussion
« Reply #32 on: July 27, 2015, 02:14:39 PM »
Do you and Voltronic both have 70ds and X-Rs by chance?

Did the new firmware seem to you to introduce noise that wasn't present before???  I assume once I upgrade firmware, I can't go back?


Offline voltronic

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Re: Tascam DR-70D - modification discussion
« Reply #33 on: July 27, 2015, 02:34:43 PM »
Do you and Voltronic both have 70ds and X-Rs by chance?

Did the new firmware seem to you to introduce noise that wasn't present before???  I assume once I upgrade firmware, I can't go back?

I have X-Q; not X-R.

I don't think the firmware update added any noise, at least not that I can hear.  What it did was raise the maximum gain for LOW by 10dB, add the MID range, and raise the minimum gain levels of LOW, HIGH and HIGH+ (although the max levels for those remained the same).  So for the three original gain levels, I suppose you could say that the average gain went up, making any self-noise or mic noise louder than it was before just because it's being amplified more.  For example, at any pot position other than the maximum, the HIGH range is is going to have more gain than it did pre-update.  I have the old / new gain levels listed on the FAQ page.  Just to be clear, those gain levels changed with the 1.10 firmware - not sure what version you were on previously.
« Last Edit: July 27, 2015, 02:39:51 PM by voltronic »
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Offline 2manyrocks

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Re: Tascam DR-70D - modification discussion
« Reply #34 on: July 27, 2015, 03:00:28 PM »
My unit is on 1.10 firmware.

Looks like the gain should be about the same from factory unit to factory unit at the highest end of each gain setting (low, mid, high, high plus)?  Would the gain be different on the modded unit?  (trying to figure out if the gain can be matched between the mod / factory unit without having them both physically side by side)

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Re: Tascam DR-70D - modification discussion
« Reply #35 on: July 27, 2015, 03:10:05 PM »
My unit is on 1.10 firmware.

Looks like the gain should be about the same from factory unit to factory unit at the highest end of each gain setting (low, mid, high, high plus)?  Would the gain be different on the modded unit?  (trying to figure out if the gain can be matched between the mod / factory unit without having them both physically side by side)

If you're on 1.10, then your most recent firmware update made all of the adjustments to the gain levels as I described.  You should update to 1.11, as there are some other fixes in there that they have corrected.

From everything I can tell, the mod did not change gain levels.  I took photos of the knob positions of my unit pre-mod and then matched them exactly post-mod, recording what levels the background noise was reading at as well as the peaks of the music.  Exactly the same before and after. 

I also repeated my open-circuit tests of recording silence at maximum gain for all 4 gain levels.  While I've since learned this is a lousy test to measure noise because it's like you have an infinite resistance on the circuit, I get exactly the same RMS levels on those files pre- and post-mod.
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Offline voltronic

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Re: Tascam DR-70D - modification discussion
« Reply #36 on: July 27, 2015, 04:52:55 PM »
The resistance isn't infinite, it's limited to the input impedance of the device.  So if that's 2K or 10K or whatever, you'd have that much thermal noise at the input.

OK, thanks for correcting me on that.  I may have read that "infinite" thing on GS and assumed it was correct, and you know what happens when you assume...

So for this particular recorder, it's 2k at the XLRs when set to MIC or MIC+PHANTOM; 10k when set to LINE.
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Re: Tascam DR-70D - modification discussion
« Reply #37 on: July 27, 2015, 05:54:04 PM »
All of my mics (except low-sens models) are designed with high output such that preamp noise is not a critical specification.

I just saw this reply.  Yes, when I've run all 4 channels, the X-Qs have a much hotter output (and the expected higher noise) than the CM3s.  I never realized that one of your reasons for making them this way was to make them less dependent on having a very quiet preamp. 

Are you saying that your mics should therefore not be used for a test like this?
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Offline voltronic

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Re: Tascam DR-70D - modification discussion
« Reply #38 on: July 27, 2015, 08:10:16 PM »
OK, to get the ball rolling I stuck my CM3s inside a pile of fleece blankets, ran my long cables to the second bedroom, and recorded 30 seconds at each of the four gain settings with the pots fully up, 48V phantom, 24/96 resolution.  The 70D was also running off my external RavPower USB battery.  Firmware is version 1.11.

The maximum gain levels for each setting are:

HIGH+:  +63 dB
HIGH:    +51 dB
MID:      +36 dB
LOW:    +21 dB

I looked at the files in RX, and found the quietest 15 seconds where there were no intermittent noises and exported those out, which is what is in the dropbox folder below.  There you will also find screenshots of the spectrogram / waveform overlay view, with RX's Waveform Statistics on top of it, as well as the spectral analysis view for each file.

https://www.dropbox.com/sh/wv262wtsca8xhbh/AAC1-wEt-7E5TDTtLIM2JNiga?dl=0

For reference, here are the specs for the CM3 mics.  At 16 dBA self noise, I wonder if these are still too noisy to be useful for this test.  At least these are mics a few of us here own so others can replicate this test on their own.

http://www.lineaudio.se/CM3.html
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Offline voltronic

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Re: Tascam DR-70D - modification discussion
« Reply #39 on: July 27, 2015, 08:36:33 PM »
You need to figure their absolute noise floor:

Sensitivity - 94 + self noise = absolute noise floor

For the CM3, that's -122dBA (maybe -118dBV unweighted).  Not knowing where the stock 70D falls, but going off of the general stats at Avisoft, it's entirely possible that the recorder's EIN is relevant.

Here's a screenshot of that section of the reference manual if it helps:
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Re: Tascam DR-70D - modification discussion
« Reply #40 on: July 27, 2015, 09:26:54 PM »
-120dBu = -122dBV, so that means that EIN is significant for the CM3.  If the weighting measures are roughly equal, total noise would increase by 3dB.

I was with you until this post - I don't understand why knowing the EIN of the recorder tells you anything about if the EIN of this or any other recorder is significant for a particular mic.  Up until this point, I thought that determination would be the absolute noise floor of the mic.  What am I missing?
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Re: Tascam DR-70D - modification discussion
« Reply #41 on: July 27, 2015, 10:18:52 PM »
It is.  If the EIN of the recorder is significant compared with the mic, then system noise will degrade.  It's a root-mean-square calculation, and of course you have to be comparing apples to apples, which means same reference (dBu or dBV) and weighting (A, CCIR, etc.)  As a rule of thumb, you want the recorder EIN to be at least 6dB below microphone noise.  If they are equal, system noise degrades by 3dB.

OK, that last part cleared things up for me.  So what you're saying is to use this mic without adding any noise into the system,  you'd need preamps at a level of quality like the sound devices 7-series that are at -130 dBV or better.  But I suppose that would be true for a lot of other mics and preamps out there.

I'm not sure if you looked at them yet, but are the files and analysis I posted going to be useful, assuming someone else replicates this pretty closely with a stock unit?
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Offline voltronic

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Re: Tascam DR-70D - modification discussion
« Reply #42 on: July 28, 2015, 03:12:56 PM »

Quote
I'm not sure if you looked at them yet, but are the files and analysis I posted going to be useful, assuming someone else replicates this pretty closely with a stock unit?

Yes.  What is interesting is the point at which the noise crosses to flat, which is about 10kHz.  That is better than I would have expected for -122dBV, so the mod could have significantly lower noise.
Well to my ears the noise level before and after hasn't changed, although I wonder if I'm hearing the more of the mic noise and not the preamp.  Jim said at some point that the input transistor noise would dominate at high gain settings, for what that's worth.

What I can tell you is that if I run a spectral analysis on any of my recordings pre-mod, there is a low-level artifact around 22kHz for all 96kHz sample rate recordings, and with 48kHz recordings the artifact moves to around 16kHz and becomes much higher in level.  This is gone post-mod.  See the spectral analysis images of my piano recordings on the first post to see what I mean.  If anyone's paying attention, that gives away the answer to the quiz. :)
« Last Edit: July 28, 2015, 05:45:04 PM by voltronic »
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Offline groovon

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Re: Tascam DR-70D - modification discussion
« Reply #43 on: July 28, 2015, 05:54:35 PM »
Hard to say exactly what that artifact could have been.  It could have been an interference signal that was present during the original recording.  Both the stock and modified opamps would have sufficiently high PSRR, and the caps added don't do anything about power supply noise or interference.  So either the source of the artifact is not related to the mod, or there was likely another change involved in the mod.

The mention of input transistors could change the noise analysis materially.  Discrete transistors are frequently used as a first-stage in front of an opamp for improved noise performance.  It takes more space though, so you might not see it as often in portable gear.  But it's a cheaper approach than paying for an expensive low-noise opamp (and solves a related problem of input topology).  Since there is gain in the first stage, the noise of the opamp becomes relatively lesser important as first-stage gain increases.

So if the 70D uses that type of input circuit, then opamp noise might not matter except at very low gain where you don't care anyway.

Jon, I'm wondering if you're curious enough yet to get one of these decks and do a tear-down? I'd be very interested in your ideas, and I'm sure many others would too.

Dave
« Last Edit: July 28, 2015, 06:01:22 PM by groovon »

Offline groovon

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Re: Tascam DR-70D - modification discussion
« Reply #44 on: July 28, 2015, 06:44:26 PM »
I don't have time, I have a backlog of my own development work.

Yes, I thought might be the case. Otherwise it might be an interesting project.

Dave
« Last Edit: July 28, 2015, 09:43:22 PM by groovon »

 

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