In a word - None.

But there are acceptable Digital Recorders. You might want to read Choosing A Digital Recorder for more on what is required of a digital recorder to do quality EVP work. That said let's examine what a digital voice recorder does and how it does it.

Voice recorders are intended for one purpose, to record human voice and do so over extended periods of time. They are great for witness interviews and keeping verbal logs of investigations. They are inexpensive, and as such are built that way with cost a primary concern. But that comes at a price which this discussion will address.

To begin with, these recorders are intended for long record times. Many see this as an advantage, but let's consider how they acheive these record times, some going as much as 100 hours depending on memory availability. Digital recorders work by taking an analog signal (the voice), and sampling it thousands of times each second to determine a digital equivalent to the instantaneous signal level at that time. The digital value of each of these samples are stored, one by one, in the recorder's memory. Later, when the recording is played back, the digital value is converted back to its analog level, amplified, and heard over the speaker or headphones.

As I said, many desire a long record time because they want to be able to record for long intervals without running out of memory. Digital recorders can extend the record time in two ways. First they can lower the sample rate, that is take fewer samples each second which simply makes the available memory last longer before filling. Secondly they can compress the audio files they create. This is a method where certain parts of the recorded file are deemed unneccessary and eliminated. Doing this makes each sample smaller, also using memory at a lower rate. But neither of these memory saving techniques is desirable for EVP work.

Consider the sample rate. Each of these represent instantaneous points in time when the sample was made. But sound is continuous, there really are no "instantaneous" points in a sound or voice. So the recorder when it replays the recording must fill in the gaps between these samples. But how does it know what to put there? Fact is, it doesn't. So it simply averages the last two points together and assumes the level should be approximate to that average. However it is quite possible to miss something there. When that happens we get what are known as conversion errors. These are alterations, changes, which deviate from the original sounds or voices recorded.

One way to minimize these conversion errors is to increase the sample rate; take more samples and the recorder has less averaging to do. It has more of the original audio data to work with. There are certain standards which apply to determining sample rate, Nyquist Point, etc. These are covered in more detail in the Choosing A Digital recorder article. For now, simply know that the higher the sample rate the more accurate the recorder will be at processing audio. And Voice Recorders generally use the lowest sample rates they can to record voice quality instead of full range audio.

A second factor is compression. Many voice recorders use varous compression techniques, some more effective than others. The problem is the more effective compression methods result in longer record times. Unfortunately they also result in a greater amount of alteration to the audio signal since more data is determined to be uneccessary, and is discarded. Later the recorder will simulate what the various algorithms assume should be there and insert its own data to fill in. Trouble is no recorder can really know what was removed, only simulations can be made to restore what sounds reasonably close to the original.

This may be fine for normal speech, but when it comes to EVP we really don't know what consititutes an EVP. We don't know how they are created. We don't even know what creates them. Yet here we have recorders which are removing and altering the data based on what we assume is normal speech. Since we don't know what EVPs are, how can we know what part of an EVP is uneccessary or can be removed? Answer is we can't.

So for EVP work we need to go to the next level. If we are going to use digital we need to sample at a high enough rate to prevent most conversion errors. And we need to forego the compression which can alter the audio we record. In short we have two options. First we can go with analog tape which, because it records the actual signal instaed of a digital equivalent, has no conversion errors and compression factors. Or if we go digital we can use a better quality digital recorder instead of one made for voice only.

Cost wise, analog tape is the cheapest way to go. You can get cassette recorders starting around $100 (less for used machines) up to several hundred dollars US. If you choose digital, these start around $200 and can go into the thousands depending on what you want. If you are serious about EVPs though, you will need to get a recorder which is reliable and not prone to the multitude of false positives and altered audio on cheap recorders. It's time to ditch the voice recorder and move up to something more suited to the task at hand.

The article which accompanies this one covers specifications that should be considered when choosing a digital recorder. It can be found by Clicking Here.

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