Why do we choose digital voice recorders instead of smartphones?
With smartphones capable of recording interviews and lectures, digital voice recorders aren’t nearly as common, or even as necessary, as they once were. Why spend a significant amount of money on a device that does what your smartphone already can for free or with a cheap app?
Of course, the answer to that question depends on what you intend on recording and how often you need to record lectures, meetings, interviews and other events. If you’re just looking to record a few lectures for school, your smartphone is perfectly capable of handling the task. But if you need to record audio on a daily basis, then you’ll appreciate the better audio quality and additional features offered by a digital voice recorder.
Digital Voice Recorders: How Do They Work?
Every sound you’ve ever heard starts with vibrations in the air. These vibrations move through the air like waves, so they are aptly called sound waves. It’s like dropping a pebble into a puddle and watching the ripples move outward from the center. These sound waves move at various speeds, which is why we hear different tones. For example, when you hit the lowest key on a piano, the low A, you create a ripple of sound waves that move up and down at 27.5 cycles per second, which is measured in hertz (Hz). When you hit the highest key, a high F, sound waves ripple out at 4,186 cycles per second.
Analog technology, such as records, captures a representation of these sound waves on physical media, and those waves are replicated when played back through an analog player. While records and record players have become popular again, analog devices aren’t very practical for recording on the fly.
To create a digital representation of a sound wave, a voice recorder picks up the audio with a microphone that converts the sound to an analog electric signal. The analog electric signal runs into an analog-to-digital converter (ADC). The ADC then converts the analog signal into a digital signal using pulse-code modulation or linear pulse-code modulation. In this process, the various points on the sound wave are given numerical values. When put next to each other on a graph, you can see the sound wave. A digital file might contain a lot of data, but you can write it to a hard drive or SD card, both of which can fit thousands of these files.
To play the recorded digital audio file, the reverse happens. The digital signal is converted to an analog electric signal that is sent along a wire connected to a speaker. The signal makes the speaker vibrate, which causes the air to vibrate, recreating the sound waves you originally heard.
Digital Audio Recorders: What Is Audio Fidelity?
Audio fidelity is the overall accuracy of the sound as represented by the digital recording. Audio fidelity of a digital audio recording largely depends on the sampling rate and the bit depth. Since most people hear with two ears, mono and stereo recording can also affect fidelity.
The sampling rate is the number of times per second audio is recorded. It helps to imagine it like a video camera, which generally takes 24 still-frame images each second. When you view those still frames one after the other in quick succession, you see movement. When these auditory samples are played one after another, you hear intelligible sound.
Most digital voice recorders sample audio with a 44.1kHz or 48kHz rate. The highest-fidelity handheld recorders sample as high as 96kHz. However, humans can’t process sampling rates over 50kHz in a meaningful way, so a 96kHz sampling rate is overkill in most situations.
The bit depth is a measure of the bits of information in each sample. For example, audio recorded on a CD has a bit depth of 16-bits per sample, which is the same as most digital voice recorders. The bit depth determines the amplitude of the sound wave. A higher bit depth means there is a greater dynamic value between the loudest part of the sound wave and quietest part of the sound wave. Your smartphone records audio at 8 bits per sample, while the highest-fidelity digital audio recorders have 24-bit depths.
The difference in these recordings is the dynamic potential between loudest sound wave and the quietest sound wave. Imagine recording a roundtable meeting – the person sitting closest to you will sound the loudest, and the person sitting furthest from you will sound the quietest. The bit depth helps interpret how noticeable that volume difference was when you recorded it. As such, it is less important in situations where audio fidelity is not a priority.
In some cases, a lower bit depth with a high-pass filter, which removes unwanted frequencies, is ideal. For example, recording a lecture typically only requires enough fidelity to be able to clearly hear what your professor is saying. In fact, too much fidelity could result in picking up unwanted noise – whispers, chatter, pages being turned, etc.
Mono or Stereo
Handheld digital recorders either record in mono or stereo. Mono recordings are made on one track, typically with one microphone. This means that when you listen to the track with headphones, you hear an equal mix on the left and right side. Digital voice recorders that record in stereo must have two microphones, which means they tend to cost more than mono recorders.
The difference between mono and stereo recording is depth. Stereo sound is more true to life because it is how your ears are designed to hear, similar to how two eyes provide vision depth a single eye cannot. For example, if your professor roams around the classroom during a lecture, with a stereo recorder you can identify where they were in the room at any given time. With a mono recording, you only hear fluctuations in the volume of the professor’s voice as they move away from and closer to the recorder.
Most stereo digital audio recorders have their microphones set up in an XY configuration – the microphone on the right is angled to the left side and vice versa. Some allow you to adjust the directions of the microphones so you can record audio with an AB configuration, where the microphones are parallel, or an ORTF configuration, where the microphones are directed away from each other.
Each microphone configuration creates a different stereo image. The XY configuration records sound with the least depth, as the microphones overlap each other. This emphasizes the middle of the mix and is best for recording a main source head on while picking up others on either side. The AB configuration provides more depth by de-emphasizing the center. For the widest stereo image, the ORTF configuration focuses almost entirely on separating the left and the right microphones with very little overlap.
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