I've made a cable to connect an SM58 and headphones to my iPhone following instructions I found on the internet a while ago but, of course, it doesn't work. Apparently I need to match the impedance between the mic and iPhone, anyone know how to do this?
Here's a link to an example of what I'm after
http://www.markertek.co.uk/Catalog/DSLR-Audio-Cables/SES-IPHONE-MIC-1
Cheers!
JB
iPhone mic/headphone cable
Re: iPhone mic/headphone cable
The issue is not impedance, but simple DC resistance.
Apple appears to use the DC voltage across the microphone (provided by the active microphone power circuit pull-up resistor) to detect when and whether a pair of headphones and/or headset are plugged in.
The OS uses the DC level rising or dropping around a threshold voltage to trigger interrupts and then the ISR makes decisions about what just happened at the jack pins.
If the microphone voltage is 0, that means a stereo headphone jack has been plugged in (not a microphone or headset) - since the sleeve shorts out the third ring and the microphone voltage.
If the microphone voltage is too high. (i.e above 2 volt or so) it means that nothing is plugged in and the internal microphone remains active.
The exact logic threshold voltage seems to vary from OS release to OS release, and/or from model to model of iPhone and iPod.
A dirty or otherwise high resistance cable can confuse the logic involved, as can an erratic insertion or removal of the plug.
Some Apps are confused by normal insertion or removal of the plug due to faulty logic they use to detect the headphone connector state.
To work properly, you must block the DC voltage from a dynamic microphone like an SM58 using a suitable capacitor (but it must be large enough not to reduce the LF response).
You must then connect a shunt resistor of the correct value from the microphone pin to ground. I've not played with this lately, but in the past I've had varying luck with resistors between 470 ohms and 2k depending on the model/OS version.
To confuse things still more, the inrush current from charging a capacitor (as needed for a dynamic microphone to keep the diaphragm from tensioning incorrectly) can create a pulse of current long enough to confuse the OS if the capacitor is too big.
This is complicated even more by Apple's use of various lengths of short pulses on this wire to remote control the volume, pause, play and track advance / back of the player from the headset.
With most integrated electret condenser capsules of the type used for telephone headsets, the microphone automatically does the right thing and draws enough current to keep the pin at the correct voltage for proper detection.
Note also that unlike earlier models, the iPhone 4 seems to have a hardware rolloff in the microphone preamp circuit for both external and internal microphones, that severely limits bass response and cannot be defeated.
Another issue to be aware of is that the iPhone 4 has TWO microphones and they interact in a complex fashion for noise cancellation. Under some circumstances, it appears that this processing may still remain accidentally enabled when using external microphones, with strange results (probably a bug).
I think Apple's officially discouraging using the microphone input for music use and moving towards suggesting a mutant USB connection to the docking connector for serious audio input to these devices (mutant USB because this type of portable device cannot provide enough power to satisfy USB bus qualify for the existing USB specification).
Of course the SM58 and similar microphones do also have an impedance problem - their output voltage is very low and you'l need a regular XLR to TS style transformer to get usable gain for most purposes. Most self powered condenser microphones seem to have plenty of output voltage, however without a transformer...
More than you probably wanted to know, but I hope this helps point you in the right direction - you'll still need to experiment a bit with the values for both the capacitor and resistor if you decide to continue...
M
Apple appears to use the DC voltage across the microphone (provided by the active microphone power circuit pull-up resistor) to detect when and whether a pair of headphones and/or headset are plugged in.
The OS uses the DC level rising or dropping around a threshold voltage to trigger interrupts and then the ISR makes decisions about what just happened at the jack pins.
If the microphone voltage is 0, that means a stereo headphone jack has been plugged in (not a microphone or headset) - since the sleeve shorts out the third ring and the microphone voltage.
If the microphone voltage is too high. (i.e above 2 volt or so) it means that nothing is plugged in and the internal microphone remains active.
The exact logic threshold voltage seems to vary from OS release to OS release, and/or from model to model of iPhone and iPod.
A dirty or otherwise high resistance cable can confuse the logic involved, as can an erratic insertion or removal of the plug.
Some Apps are confused by normal insertion or removal of the plug due to faulty logic they use to detect the headphone connector state.
To work properly, you must block the DC voltage from a dynamic microphone like an SM58 using a suitable capacitor (but it must be large enough not to reduce the LF response).
You must then connect a shunt resistor of the correct value from the microphone pin to ground. I've not played with this lately, but in the past I've had varying luck with resistors between 470 ohms and 2k depending on the model/OS version.
To confuse things still more, the inrush current from charging a capacitor (as needed for a dynamic microphone to keep the diaphragm from tensioning incorrectly) can create a pulse of current long enough to confuse the OS if the capacitor is too big.
This is complicated even more by Apple's use of various lengths of short pulses on this wire to remote control the volume, pause, play and track advance / back of the player from the headset.
With most integrated electret condenser capsules of the type used for telephone headsets, the microphone automatically does the right thing and draws enough current to keep the pin at the correct voltage for proper detection.
Note also that unlike earlier models, the iPhone 4 seems to have a hardware rolloff in the microphone preamp circuit for both external and internal microphones, that severely limits bass response and cannot be defeated.
Another issue to be aware of is that the iPhone 4 has TWO microphones and they interact in a complex fashion for noise cancellation. Under some circumstances, it appears that this processing may still remain accidentally enabled when using external microphones, with strange results (probably a bug).
I think Apple's officially discouraging using the microphone input for music use and moving towards suggesting a mutant USB connection to the docking connector for serious audio input to these devices (mutant USB because this type of portable device cannot provide enough power to satisfy USB bus qualify for the existing USB specification).
Of course the SM58 and similar microphones do also have an impedance problem - their output voltage is very low and you'l need a regular XLR to TS style transformer to get usable gain for most purposes. Most self powered condenser microphones seem to have plenty of output voltage, however without a transformer...
More than you probably wanted to know, but I hope this helps point you in the right direction - you'll still need to experiment a bit with the values for both the capacitor and resistor if you decide to continue...
M
Re: iPhone mic/headphone cable
Wow, thanks! That is one seriously in-depth answer! So you're saying it's a bit compicated right? 
Perhaps I'll invest in a portable recorder instead...
Perhaps I'll invest in a portable recorder instead...-
- john barleycorn
- Posts: 2 Joined: Mon May 02, 2011 12:00 am
Re: iPhone mic/headphone cable
It depends what you are trying to do!
if you're using an older phone or iPod, it's probably worth looking in to, especially if you like to horse around with DIY stuff.
On the current generation, you'll probably not get better results than from the built in microphone or a good quality external microphone built for it (there are many headset adapters for the iPod with electret microphones built in - some are very high quality - others roll off all the highs on purpose).
With the built in microphones and software from Apple or using third party products, the current iPhone and iPod make great casual portable recorders - we use our iPod and iPhone here for recording sessions and music lessons all the time. The LF rolloff actually is perfect for most applications of this sort as it eliminates handling noises and many other problems.
For pro recording for money - not so good, but a handy emergency backup - I bought an H4n instead for this purpose and love it.
If you're interest is casual, but you need the pro microphone look, I suggest looking into a prebuilt product that's already dealt with all the issues.
Google "KV connections" for a line of prebuilt adapters of this sort (I don't know how well they work).
Google "iRig" (from the Amplitube people) for an adapter like you're trying to build complete with matching microphone and processing software all worked out as a retail product.
Google "Studio Six Digital" for an amazing no-compromise measurement microphone and line input solution - however, it costs much more than just buying a recorder!
Hope this helps
M
if you're using an older phone or iPod, it's probably worth looking in to, especially if you like to horse around with DIY stuff.
On the current generation, you'll probably not get better results than from the built in microphone or a good quality external microphone built for it (there are many headset adapters for the iPod with electret microphones built in - some are very high quality - others roll off all the highs on purpose).
With the built in microphones and software from Apple or using third party products, the current iPhone and iPod make great casual portable recorders - we use our iPod and iPhone here for recording sessions and music lessons all the time. The LF rolloff actually is perfect for most applications of this sort as it eliminates handling noises and many other problems.
For pro recording for money - not so good, but a handy emergency backup - I bought an H4n instead for this purpose and love it.
If you're interest is casual, but you need the pro microphone look, I suggest looking into a prebuilt product that's already dealt with all the issues.
Google "KV connections" for a line of prebuilt adapters of this sort (I don't know how well they work).
Google "iRig" (from the Amplitube people) for an adapter like you're trying to build complete with matching microphone and processing software all worked out as a retail product.
Google "Studio Six Digital" for an amazing no-compromise measurement microphone and line input solution - however, it costs much more than just buying a recorder!
Hope this helps
M