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I'm planning a DIY cable to output audio and input it to another iPhone for recording.

The following combination safe and would that transfer audio?

From iPhone's TRRS jack (phone call phone) solder left (tip) and right (ring) audio wires both to the recording iPhone TRRS jack's mic sleeve? And solder ground wires too.

I've the Rode SC6 input/output breakout box. The purpose is to record phone conversation with another phone and simultaneously be able to listen conversation through headphones and speak to lav mic.

(In my country it's allowed to record phone conversations and I'd like to use cable instead of other recording options).

Thanks for your help

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closed as off-topic by Scott Seidman, JRE, laptop2d, Finbarr, efox29 Jun 12 at 20:13

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  • \$\begingroup\$ Why don't you record it internally on the phone the call is on? \$\endgroup\$ – Transistor Jul 28 '17 at 20:43
  • \$\begingroup\$ Don't they have apps for that? \$\endgroup\$ – laptop2d Jul 28 '17 at 23:01
  • \$\begingroup\$ @Transistor You can't. Applications have very limited ability to interact with the phone's telephony features. \$\endgroup\$ – duskwuff Jul 28 '17 at 23:06
  • \$\begingroup\$ @duskwuff: I don't have an iPhone - though I may have touched one accidentally once. Top ten iPhone call recorder apps: beebom.com/call-recorder-iphone-apps. \$\endgroup\$ – Transistor Jul 28 '17 at 23:12
  • \$\begingroup\$ @Transistor Those apps don't record the call directly from the phone. They work by making the call over a VoIP service, or by making a three-way call with a VoIP number as the third party. \$\endgroup\$ – duskwuff Jul 28 '17 at 23:16
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The audio output of the iPhone headphone jack is quite likely at a high enough level to overdrive the microphone input causing clipping. Here's a thread regarding the output level:

https://discussions.apple.com/thread/6522625

I suspect that the level isn't high enough to cause damage to the microphone input, but I don't accept any responsibility if you damage a phone. That said, you should be able to just wire it up and determine whether the resulting recording quality is satisfactory.

If the audio quality is not satisfactory, which is what I expect, you will need an attenuator to reduce the level enough to avoid clipping. There exist audio cables with built-in attenuation, which I'd used with other equipment for feeding line-level output to microphone-level input. Here's an example:

https://www.bhphotovideo.com/bnh/controller/home?A=details&O=&Q=&ap=y&c3api=1876%2C%7Bcreative%7D%2C%7Bkeyword%7D&gclid=Cj0KCQjwwevLBRCGARIsAKnAJvdQs-QLseQ5T1qurRHzvghWKq66e6G92sZqLCOaGRx118rVVkoBX6kaAq50EALw_wcB&is=REG&m=Y&sku=767984

I have not actually purchased that specific product, so I can't attest to its quality or suitability. The 25dB attenuation it provided could be too much. Also you will still need an adapter for the microphone plug end to connect it to the appropriate microphone and ground contacts of the iPhone TRS headphone connector.

You could also wire up your own T-pad attenuator for whatever amount of attenuation you need, made of three resistors. For instance, if you use three 20 ohm resistors, with two in series between the source and load, and one as a shunt between the middle of the series pair and ground, you'd get an attenuation of about 11.5 dB, and an impedance of about 34.4 ohms. There are various online calculators for T-pad attenuators.

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  • 1
    \$\begingroup\$ Thank you Eric for your help. I wired up a cable without attenuator. It seems that iPhone 4 doesn't recognize that output from other source and continues recording with its internal mic. I found from other thread that 1.6K resistor between mic and ground would iPhone to switch to the external microphone... \$\endgroup\$ – syreeni Jul 29 '17 at 10:31

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