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Oct 07, 2013, 07:15 PM
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Phone hacking

Having a self monitoring headset for the phone has been a long running battle. Commercial headsets don't monitor their own microphone.

Another attempt at a self monitoring phone headset failed. The phone's microphone connector appeared to be flaky, so making the headset use bluetooth was the next step.

Bluetooth sound quality was horrible. The absolute lowest samplerate is used. Microphone gain was too low. The stock microphone seemed to have much higher gain. The deal breaker was RF interference from the bluetooth chip coming out of the monitoring amplifier.

It was time to dissect the phone headphone connector.

The back is a nest of antennas. It seems to have a piezo buzzer in addition to a speaker.

1 more antenna nested in the plastic.

The headphone plug is pressure fitted with a connection that is quite good.

The problem seemed to be the tiny gold piece connecting the microphone ring. That needed more positive pressure than it was getting, to be reliable.

The final chapter in the phone adventure was the $parkfun cable itself. There was nothing wrong with the phone. The cable had a bad connection. There isn't much quality control with those.

The ideal way to solve the problem is to have the microphone loop back to the speaker on the phone instead of externally, so you can know when it's connected, but nothing that really works exists. For the recording software, there's AudioRecord which is extremely finicky & requires rebooting the phone if you get the wrong parameters. Buffer size, sample rate, input source all have to be perfect. The minimum buffer size is quite large, making for long latency.

Android seems to have an unlimited number of ways to play audio, which only depend on how long you want to Goog search. The 3 initial ones were MediaPlayer, SoundPool, & AudioTrack.

Mediaplayer has latency of several seconds. It can play a live stream if the stream is written to a fake web server & the MediaPlayer is told to play from the fake web server. People copy the file from NPR & modify the processRequest function to read from a fifo instead of a file.

SoundPool only plays short sounds, but has the lowest latency. It has to preload the sounds into RAM & there's no way to tell when it's finished playing.

AudioTrack provides the closest access to the audio hardware. It actually plays audio buffers as they're written. The latency is down in the 50ms range.

Unfortunately, the latency is too high to pass the microphone to the speaker for monitoring. Also, the microphone gain steps way up in phone mode, requiring some gain finesse to hear anyone. The most practical solution is now a simple level meter.

The LG Optimus F3 is very prone to overheating & shutting down. It can't do the audio loopback for long. There is hope it can handle a simple meter.

There's certainly potential for using the phone as an effects processor. It would need an external sound card & active cooling. It might be easiest to use wifi for it.

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Oct 07, 2013, 08:29 PM
Old retired Electronics Fart
Charles B's Avatar

remote audio

Hi Jack

Don't know if you have heard of the Jabra A210 audio remote that Hams use for
Hands free connection to the Ham rigs.

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