[X-Newbies] Re: iMac - audio input

[Alex]

On Apr 27, 2005, at 02:28, Charles Martin wrote:

> I suspect that the documentation is simply out of date and reflects an 
> earlier version of the product.

Could the doc be out of date? Of course, it happens all the time. So 
let's look at the issue of turntables and the iMic a little closer. But 
first, let's get a few irrelevancies out of the way.

(1) One of the docs you quoted states, "You must compensate by clicking 
on the EQ preset Bass Boost or turning up the individual frequencies to 
your taste". That's pure banana oil. (Unless he was talking about Final 
Vinyl v1.0, but v1.1, comprising the RIAA filter, was released in March 
2004, and the review's dateline is June 2004.)

(2) You said "the product *actually comes with* the RCA to 1/8" 
converter plug". For our purposes, we can forget the interconnects. 
Griffin does sell a specially grounded turntable cable (I'm not sure if 
it's also bundled with the iMic, and if it's what you refer to). At any 
rate, if you're handy with a soldiering iron, get some Belden #89259 
cable and good quality connectors, and make yourself a better one. But 
that's not the issue.

So what is the issue? Leaving aside lots of techie stuff, let's 
remember that the average consumer audio line level signal is roughly 
around 300mV, the average consumer mic level signal is roughly around 
2mV, and the phono MM/MC pickup signal varies from somewhere around 
0.1mV to 5mV, with some designs going as high as 6-8mV. It follows that 
any audio preamp which does something like the 40-60dB amplification 
required to bring mic level to line level will also amplify a phono 
pickup signal. So even without Final Vinyl, if properly connected, the 
iMic could get a signal out of a turntable. That's because the iMic is, 
essentially, a mic preamp plus an ADC with a USB interface.

But of course, that's not enough. Because, to restore the audio signal 
correctly, the RIAA equalization also needs to be applied to the phono 
signal.
With the iMic/Final Vinyl, the process goes roughly like this:

phono signal -> iMic (ADC -> preamp) -> digital signal -> RIAA -> audio 
file

With the dedicated system it goes like this:

phono signal -> phono preamp (-> RIAA) -> iMic (-> ADC) -> digital 
signal -> audio file

The difference may appear minor, but it's actually crucial for sound 
quality when you take into consideration what's involved.

So, can you get sound directly from the average turntable to the iMic? 
Sure, with the correct interconnects, no problem. What is a problem is 
the sound quality obtained like this. Maybe it's better than tapping 
the sound from the headphone output, but, if it is, it's not by much. 
So, if you care about your music, you definitely do not want to connect 
the iMic to the turntable rather than the phono stage preamp.

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