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How can I wire a recorder cut-in box? Answered

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So, a little background: I'm a reporter, and I do a lot of telephone interviewing. There's currently not an app that works reliably for my phone to record both sides of a call, and given that I already have a very nice standalone voice recorder (sony ICD820) that has line in/line out functionality, I'd like to have a way to run the signal from my phone into the recorder, record it, and then pass it on to a headset with a mic on it.

The problem is that the recorder only has 3.5 mm stereo in/out (TRS), with no separate mic channel, so if I connect the (TRRS - mic/l/r/ground) cable from my phone directly into the recorder, then connect my headset to the 'line out' port, it ends up distorting badly, and not recording the mic channel.

As a temporary solution I A) use two "Y" cables (one connected to the phone, and one to my headset) to split the mic from the audio channels, then B) feed the audio channels into and back out of the line in/out ports on the recorder, and then C) use a coupler to mate the two mic channels. This puts the recorder in-line on the audio channel(s) only, and routes the mic channel directly from my phone to my headset (picture attached). This works great, but it's bulky and delicate, and obviously only records the incoming side of the conversation. 

How can I wire a box that will give me the ability to "cut in" a recorder on a TRRS source, which is delivering (potentially) stereo audio channels along with a mic channel - ie, up to three channels - so that the recorder is recording all three channells?
(note: it does not matter if some/all of the channels are merged).

WOULD IT WORK TO wire a box that A) summed the Left and Right channels from the phone using resistors on each channel (see the picture in the link), and B) fed the summed audio and the mic lines into the Left and Right pins of a TRS plug, respectively, to give the recorder a stereo source? I have put together a crude diagram of what I think this could look like (note that the diagram includes a DPDT switch to 'cut in' the recorder, but I would likely omit this in the final design for simplicity).

Additional question: I have seen wildly varying info on what size resistors to use on the summing circuit, and also conflicting information as to whether I need a third on the ground (???). The simplest I have seen is just using a 1K resistor on each of the incoming channels. Given that these are just standard headphone/passive mic voltages, what size resistors would I need, and where in the circuit?

Link to the simplest 'summing box' diagram I hve found for this type of application (top image):

6 Replies

Downunder35m (author)2017-03-12

This one is free and AFAIK records both you and the guy on the other end.

This is a paid version but a free one for testing is available too.

Problem is that in some countries the recording of phone calls without consent is illegal.
So some apps might refuse to do what you need but it would be worth trying.
If in doubt try some of the lower rated apps too.
Certainly easier than a hardwired solution.

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iceng (author)Downunder35m2017-03-13

+1 Impressive !

How did you search for and find the call recorder app ?

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Downunder35m (author)iceng2017-03-13

It's called the Google Playstore, available on all good Android systems ;)

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Downunder35m (author)2017-03-12

I think your only problem are the line levels.
A handsfree headset uses common ground for mic and "speaker(s)".
Depending on the phone and OS in use the levels for the speaker might be far too high for the input of your recorder.
Before I go too far here: I am assuming a smartphone or landline phone with optional headset here...
All you need is to get the levels from the headset (or the connector to the phone to be precise) low enough so the distortions are gone.
And on the other side, for the microphone you need a tiny amplifier as it would be a capacitive microphone.
The output for the speakers of the headset should still work if the signal is just passed through.
Easiest way to try would be with a tiny mic amplifier and two potentiometers, one for the output of the mic amp and the other for the speaker.
I guess 10k Ohm should do it for the start.
But there are apps that are able to record both sides of a phone call in good quality as a standard audio file as well.

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ThomasJ126 (author)Downunder35m2017-03-12

One other thing - I have tried quite a number of apps and have not found one that will reliably record call audio. If you know of one, let me know. But most of them seem to have problems of one sort or another.

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ThomasJ126 (author)Downunder35m2017-03-12

Thanks for your reply.

A couple of things to clarify, and a follow-up question:

First, yes, we are talking about an Android smartphone and passive headset.

Second, the recorder actually receives/handles the output level from the phone fine - the distortion only came when I put the four-contact (TRRS) plug into the recorder without first splitting off the 'mic' signal. In my current setup the phone output has the mic split off of it, and then the remaining audio is passed directly into the recorder, and it records nice and clear.

Follow-up question: If the mic-phone connection is still complete, why would I need an amplifier, given that the phone (and thus whatever power it is providing to the mic) hasn't been cut out of the circuit?

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