Hi, I am fairly new to Audacity but am looking to simulate a telephone effect. The standard settings of the telephone equalization effect appear to do a reasonably good job but I’d like to know exactly what it is doing, including the default settings that it implements. Does anyone knwo what these are or where to find this information specifically for the telephone effect?
Thank you,
Ashley
The standard settings of the telephone equalization effect appear to do a reasonably good job but I’d like to know exactly what it is doing, including the default settings that it implements.
You need to understand a little about the audio spectrum and what an equalizer does… The curve shows you exactly what it’s doing, if you know how to interpret it.
The audio spectrum is represented with low frequencies* (bass) on the left and high frequencies (treble) on the right. When “experimenting” I find the “Graphic” mode is easier than the “Draw” mode. If you push-up the sliders on the left, you will boost the bass. If you push-up the sliders on the right you’ll boost the highs. The telephone preset simply cuts the low & high frequencies, leaving the middle.
…There are some other things you can do to further “damage” the sound for a more realistic telephone effect.
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[ If you want cell/mobile phoney-phone, export a copy of the audio in AMR format, (IMO use lowest AMR rate: 4.75kbps) ].
The telephone effect is an approximation of what actually happens in the Real World. The phone company used to and I think might still do produce network signaling tones at 2600 Hz. That’s where the 2600 Magazine name comes from, from amateur techies messing with the phone system.
So your voice has to be gone by then—that’s the highest pitch tone in the voice channel. It helps that your ear sensitivity happens to peak right about there.
For the same reason we apply the Low Rolloff for Speech filter in Audiobooks, nothing good or desirable happens at low voice frequencies, so you can ignore or suppress them. Couple that with telephone microphones not being particularly good at low pitch tones, and you get a low pitch rolloff at about 250Hz or so.
There’s nothing in the technical specification about low pitch tones, they just naturally don’t go through a telephone system with all the transformers and wired network matching devices.
Rounding the numbers so they’re easy to remember, that gives you a 300Hz to 3000Hz telephone filter recognized the world over.
I’m sure most of the network doesn’t work like that any more, but it may be more restricted than you think because if two people on land-lines talk to each other across the country, the whole network has to run in real time. Little or no digitizing error.
Koz
One more. The original carbon microphone in a handset is highly susceptible to compression. So for a real convincing effect, you also need to squeeze the dynamic range of the voice. Remember Leveler? That would do it.
Where did we leave it…?
Effect > Distortion > Leveler.
Koz
Koz
My “Helmet radio” plugin also does a good analog-phone impersonation …
If you need that LPC touch of a modern cell-phone, saving a copy in AMR format can add that.
Thank you all. So it looks to me like it’s just applying a bandpass filter. Apologies if I’m asking dumb questions, is that all it’s doing? If so, how can I get information like the exact filter settings (cutoff frequencies, roll off, etc.)?
Thanks again,
Ashley
how can I get information like the exact filter settings (cutoff frequencies, roll off, etc.)
You can read my post above. In the telco network, your voice has to be gone by 2600Hz, so their filters are much sharper on the high side than the low, So actual voice filtering is non-symmetrical.
This is where we ask why you need to know.
Why do you need to know? You’re not producing a theatrical voice as part of a show, are you? You would have been able to do that several posts ago.
Koz