Archive for March, 2011

Asterisk 101

For those readers of this blog who are *not* involved with the installation, developing, coding, or general hobbying it up with Asterisk (the legendary Open-Source PBX and the fastest-growing telephony platform in the world today) — your eyes may glaze over while reading this particular blog post. It’s a long-overdue primer which will discuss the properties, aspects, and care and feeding of Asterisk prompts.

I’ve been voicing prompts for the Asterisk system since the very beginning; it’s an amazing community of developers who send me prompts to voice; I record them, and then they are freely distributed to the rest of the community. It’s a cumulative building of a massive database of prompts, and the spirit of ordering  the prompts but then setting them free into the community — and benefiting from other people’s contributions — is what has made Asterisk the success it is.

Rod Montgomery

I have a confession: I, like most other professional voice talent, am not an audio expert. Sure, I’ve acquired a rather large working understanding of audio production  in the ten or so years I’ve been doing it full-time — but it’s been gained largely through trial and error, experimentation, and let’s be honest: through some mistakes. I have had a great advocate in many of the Digium staffers who have assisted in untangling various issues — none have been as helpful as Rod Montgomery, Product Manager for Digium, who has been a calm, accessible source of information and who has unsnarled many a mystery, particularly when it comes to clients of mine who are new to Asterisk. The fact that he’s also a major audio wonk on his own time is a blessing. Naturally, I knew he’d be the ultimate source of information for a blog entry which hopefully simplifies the requirements for Asterisk files, and what the likely causes are for difficulties which people may have when trying to implement them and have them play optimally.

Here’s my interview with Rod, which sheds much light on the technicals aspects of Asterisk files:

Rod, as people hear them over a phone line, what file characteristics does an Asterisk file have?

RM: Typical, plain-old telephone service transmits only a portion of the audio that is spoken from telephone to another. The human voice generates sound between 80 hz to 12,000 hz (12 khz) but a normal telephone transmits only 300-3500 hz.  Even though Asterisk supports high-quality audio, the sound Asterisk plays or records is limited by the phones in use.

If phone lines are limited to certain level of hz it can transmit, why am I asked to record Asterisk prompts in high-res (16 bit, 48,000 — actually better than broadcast quality)?

RM: Your prompts are crystal clear, and allow customers more flexibility in their application of the custom prompts. As with digital images, down-converting a high-quality file can be useful; but up-converting a low quality file yields poor results. Recording at such a high sampling rate (48 khz) and bit depth (16 bit) provides the freedom to convert other formats while retaining much of the original quality as possible in the target format. It bears mentioning: Asterisk is smart enough to play the highest-quality prompt available for the type of phone is use.

What happens if someone tries to install the high-quality sampling rate files directly into Asterisk? I understand that files don’t do well transcoding on the fly …

RM: While .wav for mat files can contain data at a variety of sampling rates and bit depths, Asterisk can rea only two kinds of these files: Microsoft WAV format at 8000 hz signed linear (with a lowercase .wav extension) or WAV-GSM, also called wav49, with an uppercase “WAV” extension. The GSM  style is usually used with voicemail records or in email attachments. If you try to play a high sampling rate format Asterisk doesn’t understand, it will throw warnings which indicate Asterisk cannot play 48K, thus cannot open the file, and furthermore, cannot find a ulaw version of the file to play.

Rod, if someone encounters a “scratchy” or “distorted” quality to the files which sound otherwise crystal clear on a standard computer media player, what are the likely causes?

RM: The most frequent mistake I see in audio production for telephony is creating files that are simply too loud. Some audio production tools will normalize files by default, saving them as “hot” as they can. This can cause files to “clip”.  Try keeping the volume levels within desire ranges and keep levels consistent; also, try lowing the volume a bit before converting to Asterisk-compatible formats.

Can anyone convert Asterisk files?

RM: Yes! Digium offers a simple web-based service to convert to .wav, GSM, signed linear, and G.729.  Also,  the <a href=”“>SoX command-line tool</a> is popular for converting files in bulk or automating file processing. It’s often available in your favorite Linux distribution.

There are many factors which can also contribute to inconsistency or problems encountered when trying to implement Asterisk files — many of which can be configuration issues on the end-user side. Asterisk is known for its user-friendly nature and its straight-out-of-the-box usability, but when issues do arise, Digium has vast resources available to help troubleshoot:

Thanks for reading! In my next post — in two weeks time — I’ll delve into the strange and mysterious world of Phone Phreakers.

Editor’s Note: The Asterisk community recently lost an integral and vibrant member: Arizona-based Asterisk installer Aaron Dahlberg was killed while vacationing in Puerto Rico on March 19th. His enthusiasm for the project was unmatched, he was a good friend of mine, and he will be dearly missed by the Asterisk Community.

Allison Smith is a professional telephone voice, who can be heard voicing systems for telephone systems and private companies throughout the world, including platforms for Verizon, Qwest, Cingular, Sprint, Bell Canada, Hawai’ian Telcom, and Asterisk.  Her website is


We’re Everywhere!

You tell her who’s boss!”

The encouraging support came  from behind me, while self-checking some groceries recently. I looked surprised at the clerk who oversees all of the self-check lanes at my local Safeway, and he gleefully continued: “You sure told HER!”

Apparently  — like most other customers who have resigned ourselves to being our own cashiers (and as a sidenote: twenty years ago, if someone had told me that sometime in the near future I would happily ring through and bag my own groceries, I would have told them they were bonkers) — I was actually answering the automated voice who guides consumers through the sometimes mystifying process of scanning our own groceries. I was responding to her specific prompt, in which she says — a little too enthusiastically, as though this is a monumental idea which just occurred to her —  “HAVE YOU SCANNED YOUR CLUB CARD??!” I — a professional voice talent, and quite an apt mimic, matched her intonation precisely as I responded: “NO, I haven’t! But I will — when I’m done! Thank you!”

You would think that myself — so frequently on the *other* side of the mic; recording automated systems which undoubtedly must frustrate other human beings, would be somewhat more accepting and accommodating to following the voice prompts which another voice talent has recorded — after all, we’re in the same line of work, and who better to sympathize with the unforgiving onslaught of monotonous instructions which the customer is bombarded with that someone who creates them herself?

And yet, there I am — expressing frustration with her for simply articulating a prompt that’s merely programmed into the system at a prescribed sequence in the process. I find it hard to hide my ire when she declares: “Unknown item in the bagging area,” when all I’ve done is place my shopping bag there for loading. (Doesn’t she know the difference between a pre-paid-for recycled shopping bag and a rutabaga?)

Maybe what sets me apart from other shoppers is that I *know* — better than most — the “she” *is* an actual human being who voiced the files which eventually became automated sound segments which play in a mechanical way…it takes an actual *person* to voice them — which the standard consumer can easily forget (or never even give thought to), and they are more readily able to dismiss her as a “computer voice.” Even prompts which are concatenated (put into a sequence) via a text-to speech utility, are initially voiced by an actual live person. When I pointed out to the Safeway supervisor that I *am* actually one of these people who voices this kind of thing, the irony of me getting snippy with the system wasn’t lost on him.

Voice talent are everywhere. Any time you encounter pre-programmed instructions “speaking” to you, they are the evidence of a voice talent’s work — and they’re often in non-traditional situations instead of the usual instances where one might hear voice talent (TV, Radio, Telephony.)

While not yet the voice of an automated grocery check-out system, I have voiced many automated prompts which are ultimately designed to facilitate transactions and make life easier — that’s the goal, anyhow. Automated banking systems, public transit stop announcements, kiosks, GPS systems, elevator floor announcements, hotel wake-up calls, parking facility ticket machines, automated taxicab change-dispensing systems, talking piggy banks  — even the talking prototype for the Rhoomba vacuum which runs on Asterisk — I’ve been the voice behind automation in many different forms and for a vast audience. While fully aware that most of the instructions which I’ve voiced simply go by, obeyed but largely unnoticed, I’m all too aware that I must sometimes generate a lot of “talkback” — much like my “dialogue” with the grocery store automaton that day.

My first experience with a GPS system was when I rented a car in Phoenix years ago to attend a convention — I, at first, articulated out loud — upon hearing the female GPS voice’s initial instructions — “Well, you’re not so great! How’d you end up getting *this* job?” She proceeded to get me lost. Rather severely. Like, “Welcome to Montana” kind of lost.  I was  in a snit about not being the voice of Garmin and disregarded her recommended twists and turns — I soon realized that the trip went smoother when I paid attention to “Trixie’s” directions (even though I eventually deferred to her, I had to diminish her by giving her a cocktail-waitress name) but couldn’t help but giggle at her frustrated tone if I didn’t take a sanctioned turn: she sounds slightly exasperated and says: “(Sigh!) REcalculating…” Another favorite is my Bluetooth utility for my car, with a British-accented female voice who sounds like she’s saying “Bugger off!” when she’s saying “Power off!” No, YOU Power off!

Of course, it needs to be pointed out that the prompts have a life of their own — they’re just following the sequence dictated by the computer which runs it; we, as the voice talent, are rendering absolutely no judgement when you entered the code for bananas but didn’t leave them long enough on the scale for weighing; we’re not actually admonishing you when your pin numbers do not match, and there’s no actual commentary behind the pre-programmed message which might tell you that you’re in your overdraft or that you have insufficient funds for the transaction you were planning to do. It may sound very much like another human being is getting on your case — but that’s just a construct of “personalizing” a system which is aurally-driven rather than visual.

And when the Park n’ Ride ticket dispenser issues my ticket when parking at the airport, I’m known for thanking “him” with a hearty “Row J it is! And thank YOU, sir!”

I figure he gets enough abuse from everyone else.

Join me in two week’s time (remember: the blog is now bi-weekly) for an article that’s needed to be written for quite some time: many newer users of Asterisk are hazy about the file specs, the optimal settings, and the basic care and feeding of Asterisk prompts — with the help of Rod Montgomery of Digium, I’ll be writing a much-needed primer on “Asterisk 101”!

As always, thanks for reading, and your feedback is always welcome!

Allison Smith is a professional telephone voice, who can be heard voicing systems for telephone systems and private companies throughout the world, including platforms for Verizon, Qwest, Cingular, Sprint, Bell Canada, Hawai’ian Telcom, and Asterisk.  Her website is