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​Working Out the Audio Path

A project log for An Old Fashion Acoustic Modem for the iPhone

My son recently returned from a holiday in China. His biggest complaint was the blocking of FaceBook!

agp.cooperagp.cooper 01/22/2017 at 01:570 Comments

Working Out the Audio Path

Microphone Estimate Check

Here is my estimate for speaking into a microphone 25 mm away:

Speaking into a microphone Estimate
Voice at 1 m60.0dB SPL (1 W / 1 m)
No Power Adjustment0.0dB
SPL (uP) -> V rms (the magic number!)-74.0dB
Microphone Sensitivity-63.0dB
Microphone Rated at 1k Load0.0dB
Assume an effective 25 mm mouth to microphone spacing32.0dB
Amplifier Gain (x1)0.0dB
Voltage (rms)-45.0dB
Voltage (rms)5.623mV rms
Voltage (pp)15.905mV pp


So is the mathematics right?

Jack Smith (www.cliftonlaboratories.com/microphone_output_level.htm) piblished:

"The image below is at a speaking level more typical of what I would use in a normal QSO. Although there are a few peaks of 40 mV or more, the majority of the speech is around 20 mV peak-to-peak."

So my estimate is reasonable.

iPhone Output Level

Next, what is the output level of the iPhone in normal handset mode?

Hard to find on the Internet but a "dial tone" was quoted at 80 dB. This sounds okay, 80 dB is the level where you raise your voice to be heard and beyond that you risk long term hearing damage.

On that basis I can expect 4 vpp from the microphone (assuming I use the standard 1k load resistance, the microphone sensitivity is proportional to the load resistance within limits):

Dial Tone Estimate
Telephone Dial Tone80.0dB SPL (1 W / 1 m)
No Power Adjustment0.0dB
SPL (uP) -> V rms-74.0dB
Microphone Sensitivity-63.0dB
Microphone rated at 1k load0.0dB
Assume an effective 10 mm speaker to microphone spacing0.0dB
Amplifier Gain (x1)0.0dB
Voltage (rms)-57.0dB
Voltage (rms)1.413mV rms
Voltage (pp)3.995mV pp

Here I have had to assume an effective speaker to microphone distance of 10 mm.

Modem Speaker to iPhone Estimate

Here I have made two calculations: the first is normal speaking into an iPhone and the second the modem. The main difference is the power and distance from the "speaker" to the microphone:

Voice to Iphone
Speaker Sensitivity60.0dB SPL (1 W / 1 m)
No Power Adjustment0.0dB
Distance Correction (50 mm)26.0dB
Sound Level86.0dB
Speaker to Iphone
Speaker Sensitivity80.0dB SPL (1 W / 1 m)
Power Adjustment (4 mW)-48.0dB
Distance Correction (10 mm)40.0dB
Sound Level72.0dB

There is a difference af 14 dB (x5) but this is well within the AGC range of the iPhone.

So the current design is workable.

iPhone to Modem Microphone Estimate

This question is more about the amount of gain required for the modem microphone:

Modem Receive Estimate
iPhone Output80.0dB SPL (1 W / 1 m)
No power adjustment0.0dB
SPL (uP) -> V rms-74.0dB
Microphone Sensitivity-63.0dB
Microphone rated at 1k load0.0dB
Assume an effective 10 mm speaker to microphone spacing0.0dB
Amplifier Gain (x200)46.0dB
Voltage (rms)-11.0dB
Voltage (rms)0.283V rms
Voltage (pp)0.799V pp

As the target average uP signal is about 0.7 vpp in order to maximise the dynamic range (5 v down to 0.1 v), a microphone gain of x200 is required. An LM386 would work well here.

The AM4011 Microphone

Checking the bias current for the AM4011 microphone:

Voltage across microphone Microphone Current
2.2 v0.31 mA
4.4 v0.32 mA
7.7 v0.36 mA

Based on the above measurements, a 5v supply and a 10k load resistor will work fine (approximately 2 volts across the microphone). This suggests a 20 dB gain improvement over the 1k load resistor test condition.

This suggest I will have to reduce the microphone amplifier back to maintain 1 vpp:

Modem Receive Estimate
iPhone Output 80.0 dB SPL (1 W / 1 m)
No power adjustment 0.0 dB
SPL (uP) -> V rms -74.0 dB
Microphone Sensitivity (rated at 1k load) -63.0 dB
10k Microphone Load Resistor 20.0 dB
Assume an effective 10 mm speaker to microphone spacing 0.0 dB
Amplifier Gain (x25) 28.0 dB
Voltage (rms) -9.0 dB
Voltage (rms) 0.353 V rms
Voltage (pp) 0.999 V pp

We will see if the microphone amplifier can be dropped back to a gain of only 28 dB!

Microphone Test In-Circuit

I assembled Modem V2 and checked the microphone circuit. The microphone worked well but only had 0.9v across it? Checked the circuit for wiring errors and incorrect component values, no faults found. Took the microphone out and checked it again (2.17v with a 10k resistor). Now I know the power supply for the test is 5.22v and the Nano gives out only 4.69 volts and I included a 1k resistor in series with the 10k resistor for the decoupling capacitor but 0.9v, really? Eventually I worked out that the decoupling capacitor was bad so I replaced it. I decided to bump up the microphone voltage by replacing the 10k resistor with a 6k8 resistor. The microphone voltage is now 2.15v in-circuit. I have another microphone which measured 1.8v in the same out of circuit test, so good to be above 2v in -circuit.

With speaking normal level voice at 0.5m I am getting about 0.5vpp. My dog barking at the rubbish truck 3m from the microphone pull 5vpp.

A 500uW signal from the modem speaker pulls 1vpp at 1cm.

AlanX

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