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?