Topic: Quick Bass Rolloff question...

[inside source]

alright, thanks, sorry about this silly question.. 

[Genghis Cougar Mellen Khan]

I assumed that MD has 20 kOhm line input impedance, please correct me if I'm wrong. Is it correct for the new Hi-MD recorders? I didn't find it in the manual.

Why would you assume that?  I'd guess that it's 47k Ohm like previous recorders.  Does anyone know for fact what the line/mic impedence for the new Sony Hi-MD recorders is?

[audBall]

Mic-in frequencies (based on the pcm-m1 Mic-In resistance of 4.7 kOhm) :
  (16Hz) >  32Hz    2,3,4   
  (69Hz) > 138Hz   1,4   
  (95Hz) > 190Hz   1,3   
(107Hz) > 214Hz   1,3,4   
(160Hz) > 320Hz   1,2   
(195Hz) > 390Hz   1,2,4   
(888Hz) > 1776Hz 1,2,3   

Line-in frequencies (based on the pcm-m1 Line-In resistance of 47 kOhm) :
(16Hz) > 3.2Hz    2,3,4   
(69Hz) > 13.8Hz  1,4   
(95Hz) > 19Hz  1,3   
(107Hz) > 21.4Hz  1,3,4   
(160Hz) > 32Hz  1,2   
(195Hz) > 39Hz  1,2,4   
(888Hz) > 177.6Hz  1,2,3

Also valid for the d7 (or any other unit with a line-in resistance of 47 kOhm)

Could I make this same assumption with the JB3 (that going Mic-In would be a 10-fold increase in bass roll-off at the default db setting that mic-in provides??...i can't remember what that is right now). 

[dklein]

no, you'd have to measure it...but it doesn't really matter because the mic-in severely rolls off the high frequencies, rendering it useless for music recording.

[audBall]

no, you'd have to measure it...but it doesn't really matter because the mic-in severely rolls off the high frequencies, rendering it useless for music recording.

Thanks for the note...now I can definitely see why the roll-off option on the batt box is pointless for the JB3.  Looks like my switches won't get used now 

Although, could it be useful in conjunction w/ the AD20 (rolling off bass and increasing gain) effectively?  I'm planning on catching a hiphop show(Digable Planets) this weekend and would like to get somewhat of an idea of how to do this with the most desirable results.  Also, Kottke/Gordon is a couple days after.  Any suggestions?  I do have the phantom option, but I do not want to stealth with it.  Both shows are at the same venue and I don't think I could get the phantom in anyways (they pat you down, make you lift your belt area, etc...BS)

[inside source]

I assumed that MD has 20 kOhm line input impedance, please correct me if I'm wrong. Is it correct for the new Hi-MD recorders? I didn't find it in the manual.

Why would you assume that?  I'd guess that it's 47k Ohm like previous recorders.  Does anyone know for fact what the line/mic impedence for the new Sony Hi-MD recorders is?

As I stated I'm not sure. My assumption is based on what I found at minidisc.org forums - 20 kOhm impedance for line-in for both MD and Hi-MD models. I've seen 47 kOhm was also mentioned somewhere else in the internet. It would be great if someone could measure it and give us a definite answer. Unfortunately this characteristic is not listed in the minidisc manuals under specifications.


Also on this subject, below is a quote from someone at minidisc.org forums posted a while ago. It is contrary to all the posts here and I had asked for further clarification, but didn't get the answer. So what do you guys think?

- - -
Input impedance does not have any effect on the actual cut-off frequency of a filter. Fact is, only the filter itself is based on an impedance that bears an effect on the frequency being fed through it.

In the case of the input impedance of the recording device, the last thing we want is for the input to function as a filter as well and so by design, the impedance does not have an effect on the unit's intended frequency response hence the reason why mismatched impedances do not affect frequency response directly.

In theory, the input of an operational amplifier has infinite impedance to prevent loading of the output device feeding it the input signal whilst the output is of zero impedance to ensure maximum power is fed out of it. So it is this operational amplifier and its resistors which contribute to its overall input impedance - not a network of caps and inductors (as caps would be used for DC blocking purposes but nothing more).

What does affect mismatched impedances, however, is the maximum power transfer which affects the entire frequency spectrum as we're now talking about an amplitude related phenomenon.

So irrespective of what the bass roll-off filter's characteristics, whatever properly designed input it is plugged into, the characteristics of the filter remain the same leaving only its overall output power transfer affected.
- - -

[cgrooves]

Interesting quote to say the least.  As you stated, it is contrary to what has been stated here.  My knowledge is limited, however, I have discussed this issue with 2 different individuals that build battery boxes.  Both of these individuals informed me that their boxes were designed 'based on a generic impedance of 10 KOhms'.  Both of these individuals also stated that I would need to correlate the actual roll-off values, as based on the input impedence of the recording device used.

Most of our recording devices (if not all) do not utilize the 'generic impedance of 10 KOhms'. 
Like I stated above, this is only what I have been told by people I believed to be knowledgable about the subject. 

[dklein]

Poppycock!  First off, is this guy talking about passive filters?  It's not that the input impedance of the recording device is acting as a filter.  The DC blocking caps used in the battery box are effectively 'too small', which causes the desired rolloff.  The input impedance becomes the 'R' part of the RC circuit.  While an opamp has theoretical infinite input impedance, that's not what you get when you build a preamp circuit with it.

Clearly this person has some technical knowledge - perhaps they didn't understand the specific application.

Or maybe I don't know what I'm talking about...but then there's that real world experience...I've played with these circuits, different recorders, different cap values.  Maybe I'm not explaining it properly but it's not theoretical!  And this RC stuff is pretty basic electronics (not to say it's simple but you run into it everywhere if you're trying to learn the basics like I am).  Google it!

[inside source]

yeah, dklein, the application of this theary to this specific situation is exactly what I was questioning the original poster at minidisc.org.
Here is that thread: http://forums.minidisc.org/index.php?showtopic=9095  - the quoted post is #14.

Meanwhile, does "bass roll-off at 100 Hz" mean that the signal starts to decay at 100 Hz linearly and the signal amplitude is reduced by 6 dB at 50 Hz?

[inside source]

Both of these individuals informed me that their boxes were designed 'based on a generic impedance of 10 KOhms'.  Both of these individuals also stated that I would need to correlate the actual roll-off values, as based on the input impedence of the recording device used.

In this case the available bass roll-off settings on SP battery box make me wonder why they didn't include higher frequencies roll-off (between 195 and 888, so it could be used with DAT/HDD recorders which have up to 3-5 times higher input impedance. Or at least some kind of switch that would adjust for a higher input impedance.

Oh by the way I totally forgot about the formula for bass roll off calculation from the well-known Giant Squid FAQ from which I learned about the bass roll off in the first place. I think I should quote that one here as well for for completeness sake:

- - -

Q:  What is "bass roll off?"
 
A:  Bass Roll Off simply decreases the volume of the bass by using two capacitors within the circuitry.

Decreasing the bass can result in a better balance of highs and lows during playback if field recording is made in loud, big, boomy bass environments.

Bass roll off does not eliminate or cut off the low frequencies. 
 
Different input impediances of different models of recorders will give different roll off.
Here is a mathematical formula for figuring out what kind of roll you will get with different capacitors.

1/(6.28318 x Farad of the capacitor x Ohm of the recorder's input) = starting point of roll off.

The capacitors we use are .068 micorfarad.

Unlike some of our competitors we use the larger, higher quality Polypropylene Capacitors in our battery boxes that are designed for audio low frequency bypass.  As a result Giant Squid battery boxes delivers smoother roll off and low distortion.

- - -


ok, now finally using this formula I get 234 Hz as the starting point of roll off for 10 kOhm recorders's input, which is about 50 Hz for 47 kOhm. For Giant Squid battery box, of course.

[dklein]

Meanwhile, does "bass roll-off at 100 Hz" mean that the signal starts to decay at 100 Hz linearly and the signal amplitude is reduced by 6 dB at 50 Hz?

It's the -3dB point.  See the attached

[inside source]

Thanks a lot, dklein!
That were my doubts. So in fact it's -3 dB point and the decay starts much earlier. Looks like for "bass roll off at 100 Hz" we have about -1 dB at 200 Hz already.

[leegeddy]

Thanks a lot, dklein!
That were my doubts. So in fact it's -3 dB point and the decay starts much earlier. Looks like for "bass roll off at 100 Hz" we have about -1 dB at 200 Hz already.

.....but in practicality, can you REALLY distinguish a 1dB drop at 200hz in a recording given an already noisy environment?

to add to the discussion about rolloff filters, you could always design a higher order filter (say...12dB/octave or 18dB/octave) which will give you a sharper rolloff. that would involve using an RLC circuit.

marc

[inside source]

What about Edirol R-1? It has 100 kOhm line-in impedance. Is anyone using "bass roll-off at 888 Hz" (read 88.8 Hz) with it?