|EVRC: The Savior of CDMA?|
Last Updated: 15-Apr-2004
Before we begin our discussion of EVRC, let’s first look at what a CODEC is, and what role it plays in digital communication. A CODEC is essentially a data compression algorithm that selectively throws away information that it deems to be unimportant, while retaining the data that it deems necessary. The idea is to stuff as much information into as small a stream of data as possible.
However, the CODEC plays another important role in the communications link. It must be robust enough to withstand a certain degree of data degradation during transmission. We refer to the degree of damage as the Bit Error Rate (or BER for short). The better the CODEC can recover from bit errors, the more natural the reproduction will be over a wide range of conditions.
The original CDMA CODEC was not particularly robust, and as such it made even low Bit Error Rates audible to a discerning listener. Since CDMA strives to maintain the lowest possible transmit power, it is always on the verge of suffering from devastating levels of Bit Errors. For that reason the original CODEC rarely ever sounded its best, and that put a lot of people off of CDMA (myself included).
Beginning in late 1999 and early 2000, CDMA providers began implementing a new CODEC called EVRC (which stands for Enhanced Variable Rate CODEC). Their reason for implementing it was not really to provide their subscribers with better audio, but to stuff more of them into the same bandwidth. But how is that possible?
CDMA differs from all other digital standards in that each user consumes a variable amount of the “ether” according to the number of bits he or she transmits each second. The original CODEC used variable rates, but it normally transmitted at a rate of 13 kilobits per second. When the CODEC was first designed you needed at least 13 kilobits to achieve reasonable audio quality. However, EVRC manages to produce reasonable audio quality in only 8 kilobits. Since EVRC users consume less ether than users of the old CODEC, a CDMA “carrier” can accommodate more of them.
It isn't just that EVRC masks bit errors more efficiently, it is also the way that EVRC responds to them that makes it better. The old CODEC would often manifest Bit Errors as odd frequency shifts in the voice of your caller. This gave the audio a decidedly electronic sound. EVRC on the other hand simply produces short breaks in the audio, or slight "rustles" that don't degrade the overall sound of the reproduction. When Bit Errors become too great, you will often hear longer blanks in the audio, or soft buzzing sounds. The overall effect is much more pleasant, however.
I was initially skeptical that the improvement would be all that noticeable, but I was really impressed by the difference (in terms of stability). I tested EVRC using a Motorola Timeport on Clearnet PCS, and my experience with it was very positive. In prolonged calls that I made while driving around Mississauga and Toronto I could rarely detect any audible effects from Bit Errors, so long as it wasn’t rush hour. The Bit Error Rate during rush hour sometimes climbed much higher than EVRC could cope with in a few areas of the GTA. Clearnet might be able to help there by implementing more “carriers” (CDMA channels).
Before CDMA was released to the public in 1997 I had very high hopes for it based upon the hype generated by Qualcomm. When I finally got a chance to try CDMA however, I was totally floored by the audio problems. Until the switch to EVRC on the part of Clearnet and Bell Mobility I had never been able to stomach CDMA as an alternative to the trusty GSM service. The overall quality of EVRC, coupled with CDMA’s complete lack of audible handoffs, temped me to switch, but all was not perfect.
8 kilobits is still not enough to reproduce crisp audio like a well-engineered 13 kilobit CODEC (such as EFR on GSM). Depending upon the source material you'll find EVRC sounds less crisp (especially in the "s" sounds) and a little raspier overall. However, under many circumstances this isn't all that great a difference. Unfortunately that isn't the only problem. One of EVRC's claimed "advantages" is built-in noise suppression. That seems like a great idea until you hear what it can do to audio quality, especially outgoing audio.
As the background noise increases (like in noisy cars travelling at highway speeds) the noise canceller knocks out increasingly large chunks of the sound. In doing so it damages the audio that you do want (I.E. your voice). I've had many friends complain about the quality of the audio when I've phoned them from my wife's noisy pickup truck, but they say I sound great from my relatively quiet car. When I make the same call from the noisy pickup truck using non-CDMA phones my friends tell me they can hear the background noise, but my voice sounds clean and undistorted.
Finally, EVRC just doesn't work well if your caller happens to be using a digital cell phone. It's "okay" if they are also using a CDMA phone on the same network, but it sounds horrible if they are using a GSM or iDEN phone (even if those callers all sound terrific on a landline phone).
On balance however, I would say that EVRC is a step in the right direction for CDMA. Except for a lack of overall crispness and clarity, it is an worthwhile improvement over the earlier 13 kilobit CODEC that it replaces. How much the problems I've mentioned above will bother you depends on how much sound quality matters. If you aren't an "audiophile" when it comes to mobile phones, I think you'll love the stability that EVRC brings to CDMA.
All of Clearnet's phones, except for the original Sony model, support the EVRC CODEC. If you own a Clearnet phone, then EVRC will be selected for you automatically. However, not all phones implement EVRC as well as others. My own experience has been that the Nokia 6188 seems to have the least capable implementation. The Motorola Timeport is by far the best, and all the rest come somewhere in-between those two extremes.
Bell Mobility now officially supports EVRC. Unless you have a phone that doesn't have EVRC (like the NP-1000, of any of the older Qualcomm models) then you'll get EVRC automatically. In either case, there is NO WAY to turn it off if you don't happen to like it.
How Do You Know if you have EVRC Enabled?
The only phones on which I know that you can determine absolutely if EVRC is available is the Nokia 6185 and Nokia 6188. On these phones, put the unit in Field Test Mode, and go to screen 01. While making a call, observe the 4-digit number on the second line. It will be one of the following 3 values:
Standard 13 kilobit CODEC
0001 Standard 8 kilobit CODEC
For all other phones, you are going to have to play it by ear. It is very difficult to explain what to listen for such that I could guarantee that you'd know for sure. However, the following guidelines should help:
- EVRC is much more robust, and its audio degrades far more gracefully than either of the older CODECs. In the face of relatively high Bit Error Rates, you'll hear short dropouts, but the audio quality should remain constant. With the old CODECs, the audio would often take on a very weird flavor in the face of even small numbers of Bit Errors.
- The old CODECs often experience what some people have called a "digital fart". It's a loud, short duration, noise that hits you at random. EVRC never suffers from that phenomenon. Not hearing such a sound doesn't prove you have EVRC, but hearing one proves you don't.
- When the Bit Error Rate gets really high, EVRC substitutes a soft buzz (or dead air) in place of the missing audio. The standard CODEC just presents a horrible distortion of the audio.
- You can drive all over the place without hearing a single audio problem with EVRC, but the standard CODEC simply cannot achieve such excellent performance. Sometimes it can do so if usage was very low, and there are very few sites covering the area you are in, but for most urban situations, it is virtually impossible.