Harmonic noise introduced when using an iPhone dock extension cable connected to a mic peripheral I am using a Tascam iM2 stereo mic connected to the dock of an iPhone 4S. When the peripheral device is connected directly, there is no problem. Hoever, when I use a dock extension cable, a strange harmonic noise is introduced. I have tried extension cables from different manufacturers, and even the highly rated dockXtender extension cable made by CableJive introduces the noise. 

This is somewhat strange as the analogue-to-digital (bear with my British English) converter is built into the peripheral device. This is not analogue noise picked up in the cable as the signal is pure digital on route to the iPhone. The noise seems to be introduced in the digital domain. I recorded a sample of audio in my office and analysed it using Matlab. The noise fluctuates in amplitude, but it is clearly a harmonic complex with a very precise 1-kHz fundamental frequency. The ADC reportedly samples at 44.1 kHz, so it is particularly strange that the harmonic noise comes in with a periodicity that is not even an intiger multiple of the sampling period. As a side note, I have heard a similar harmonic noise in the past emitted from some laptop sound cards (specifically Vaio C1s models).



Given that the noise never fluctuates in frequency, testing has shown that I can remedy the problem by pre-processing with a fractional delay filter based on the information here . . . 
http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=951433
. . . but this is a computationally expensive filter to implement.

Prevention is better than cure. My question is, what is the origin of this strange harmonic noise and if possible, how do I remedy it so that I do not have to waste processing power implementing an additional filter?
What you are picking up is EMI from USB frame sync (most probably through the ground wire):


  Every millisecond (12000 full-bandwidth bit times), the USB host transmits a special SOF (start of frame) token, containing an 11-bit incrementing frame number in place of a device address. This is used to synchronize isochronous data flows. High-bandwidth USB 2.0 devices receive 7 additional duplicate SOF tokens per frame, each introducing a 125 µs "microframe" (60000 high-bandwidth bit times each).
  
  USB 2.0 added a PING token, which asks a device if it is ready to receive an OUT/DATA packet pair. The device responds with ACK, NAK, or STALL, as appropriate. This avoids the need to send the DATA packet if the device knows that it will just respond with NAK.


This could be avoided with a properly shielded extension cable. Note that even dockXtender only shields some of the wires:


  A revolutionary 2-tier shielding system puts all video and audio signals inside a separate shielding to reduce interference from the other signals traveling through the wire.


If you're the DIY type of person it should be quite easy to make your own extension cable using high quality shielded wires. You will probably only need to connect just a few of the 30 pins: USB (power, gnd, data+/-), all GNDs and maybe accessory/serial (enable, rx/tx). Make sure you also shield the soldering on the connector.