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ac vs dc question

2003-07-26 by grotechef

hello everybody!

i'm having trouble understanding the difference between ac and dc 
waveforms; and also between ac or dc coupled waveforms, and how this 
affects functioning of certain modules (particularly the '110 
vca/ringmod).

i've been reading a lot of manuals - arp 2600, emu, serge gold book - 
but alas, i've managed to get more confused ;o)
so, i was hoping some of you wizzards could help me out with this.

(i'm also confused about "waveforms with a dc offset/component")

i do understand that ac is alternating current, though ;o)

is there some relationship between 
ac = audio freq voltage 
dc = control voltage?

though some audio freq are not 'alternating'?

thanks!
greetings,
chef!

Re: [motm] ac vs dc question

2003-07-26 by jwbarlow@aol.com

In a message dated 7/26/2003 8:13:54 AM Pacific Daylight Time, grotechef@... writes:

Show quoted textHide quoted text

(i'm also confused about "waveforms with a dc offset/component")

i do understand that ac is alternating current, though ;o)

is there some relationship between
ac = audio freq voltage
dc = control voltage?

though some audio freq are not 'alternating'?

As far as I know the only audio signal that is not in some way alternating (periodic) would be noise as it is by definition a random waveform.

DC can't be audio. It can't be heard. You are right in that it is used for control voltages -- one of the most important developments in the early days of modular development was the "standardization" of DC voltages for use to control other modules. The 1volt/octave standard is the best example of this.

AC within the range of 20 Hz (20 cycles per second) and 20 KHz (20 thousand cycles per second) is audio. When you hear a pitch coming from a speaker there is an AC voltage across that speaker that is with in that range. AC waveforms lower than 20 Hz and higher than 20 KHz cannot be heard directly, though waveforms lower than 20 Hz can be felt.

AC signals can be easily represented on graph paper (this is especially true of the four basic synth waveforms -- sine, triangle, pulse, and sawtooth). This is useful to determine whether there is any DC offset (a DC offset is an upward or downward shift of an AC signal). If you look at page 29 of the ARP 2600 manual you will see the four waveforms drawn showing the amplitudes as they come directly from the output of VCO.

Look at the examples of the sine and triangle waveforms. Notice that the lowest point of those waveforms is at -5 volts while the highest point is at 5 volts and each of these waveforms is symmetrical about 0 volts -- the sine and triangle waveforms from the 2600 VCO 2 have NO DC offset.

Now look at the sawtooth and the pulse waves. The lowest point of each wave is at 0 volts and the highest point is at 10 volts. These waves are symmetrical about the level of 5 volts -- the pulse and sawtooth wave forms have a DC offset of five volts.

All these waveforms have the same peak to peak levels (10 volts) and the only noticeable difference between these waves will be due to the harmonic content of the wave -- the waveforms with more harmonic content will sound louder. As an audio waveform, one cannot hear the presence of a DC offset. The only way DC offset can be heard is when an audio waveform is used as a modulation source.

If you read page 30, they mention why they chose these levels. Sine and triangle waves might be used for pitch modulation, vibrato for example, of another VCO where the pitch would be expected to go up and down around a particular pitch. A pulse wave might be more useful for a tremelo effect where the pulse output is applied to the CV input of a VCA to produce a full on, full off effect of the VCA.

The MOTM 110 is AC coupled -- it will not pass a DC signal. the majority of uses for a VCA in a modular synth are to control an audio signal. The MOTM 110 is perfectly suitable for this type of use. However, there are times you will want to gate a control voltage which may have a DC component (offset), and the 110 won't work for this application though the MOTM 190 will IIRC.

Finally:
All audio signals (except noise) are AC.
All DC signals in a modular synth are control voltages of some sort.
AC signals can be used as control voltages (for complex FM for example), and can be outside the range of human hearing when used this way (vibrato is well below 20 Hz)

Get the Allen Strange book, "Electronic Music"

Hope that helps,
JB

Re: ac vs dc question

2003-07-27 by grotechef

hi jb,
and: wow! thanks!! this has been very clear! thanks for writing this!

i did find something 'strange' in the 2600 book, while searching for 
the text you mentioned in your reply.

on page 39 (ring modulator) of the arp 2600 book there's a drawing of 
ac-coupled *low frequency* square and saw waves (these are dc 
normally, right?). 

these do not look like i would have expected.

it seems like the squarewave has been shifted partly upward and 
partly downward, and the saw has been in an accident :)

is this because they are low frequency waveforms? 

> The MOTM 110 is AC coupled -- it will not pass a DC signal. the 
majority of 
> uses for a VCA in a modular synth are to control an audio signal. 
The MOTM 110 
> is perfectly suitable for this type of use. However, there are 
times you will 
> want to gate a control voltage which may have a DC component 
(offset), and the 
> 110 won't work for this application though the MOTM 190 will IIRC.

so, i can't use the 110 to vc an enveloppe (which is a 100% dc 
voltage)(eg into audio in of vca). (the eg cannot be morphed into an 
ac waveform, right?)

but using a pulse lfo instead of an enveloppe would work because the 
pulse gets ac-coupled by removing some dc-component/offset, makin it 
an ac-waveform?

(is this correct?)

> Get the Allen Strange book, "Electronic Music"

i will. i found this on the web:

"My text, Electronic Music, has been reissued by McGraw-Hill Primus
Editions. It may be difficult to get in Europe so if anyone wants a 
copy
please contact me direct at <astrange@...>

I can send you copies at my cost + shipping/etc which should be well
under $40.00 US each 

Cheers-

Allen Strange "

greetings,
chef!