Introduction to Digital Audio Recording

While digital audio recording technology continues to progress in quality and capacity the successful application of the technology is still largely dependent on how you use it. Regardless of how good your digital recording equipment or post production software is the quality of a recording is largely determined by a good recording level and microphone placement. However, this can sometimes be difficult with small level displays or awkward recording set ups. The key is to have the patience to test the equipment and monitor the levels beforehand.

The Equipment

When digitally recording an interview there are several options regarding what technology to use.

Digital Recorders such as a Zoom H1, Zoom H2, and Edirol R-09 have excellent built in internal microphones designed to record without the use of an external microphone. These stand alone units are compact and easy to use and have adequate storage capacity.

Digital recording units such as a Marantz Professional Solid State Recorder 671 ideally require a quality external microphone with a built in pre-amp, cable and stand or a lapel microphone with wireless set up.

Another portable recording method is to use a USB microphone with a laptop and sound editing software. This may save time on transferring files but will still requires a laptop with a good soundcard, adequate memory and storage space for real time digitization. Regardless of the equipment the most important requirement of any digital recording device is a way to monitor the specified strength the signal input level (voice).

 

The Basic Terminology

In order to achieve a good recording level we first need to know the basic terminology used to describe audio levels.

Dynamic Range – The captured space between the lowest level or the noise floor and loudest sound level or the peak.

Signal to Noise Ratio (SNR) – is the level of a desired signal (sound or voice) to the level of background noise.

The Permitted Maximum Level or PML - is the limit of the signal (voice) before distortion. If the signal (voice) exceeds the limit it becomes distorted and is referred to as peaking or clipping. In digital the limit is often anything beyond full scale (-0db).

Headroom – is the space between a peak signal and the PML. In digital anything below full scale (-0 db) and (-12 db) is considered good headroom.

Gain – is the measure of the ability of an amplifier to increase the power or amplitude of a signal (voice). Often measured in decibels (db) Gain isthe term most commonly used to describe adjusting the input or output levels on an electronic device.

 

What Can Go Wrong:

When recording a signal (voice) if your input level is set too low it will result in a poor signal to noise ratio with a noticeable amount of noise floor (hiss and hum) in the recording. If you exceed the PML this will result in Peaking or Clipping. This is usually indicated with a red light. If you’re using colored level meters, green is ok, yellow is still ok but approaching PML, and red indicates clipping. After the interview, it is possible, with audio editing software, to boost the levels if originally recorded too low. However it is often notpossible to restore a distorted signal (voice) due to clipping.

Getting the best level

To attain the best recording level, you should set your recording level as close to the permitted maximum level as possible, without reaching and exceeding that limit. Digital audio can come very close to the threshold for PML, without distorting. Recording levels for an interview shouldbebetween -12 db and -6 db which usually allows for enough headroom to account for irregular spikes in peak levels. The unpredictable nature of Oral History interviews can sometimes be a factor in the recordings. Conversations can be very quiet and then quickly escalate in volume. Because of this, you need to make sure to monitor recording levels throughout the recording once they’ve been set. Only change the levels during the interview if it’s completely necessary. If clipping occurs, don’t panic, but do gently back the levels down. Refrain from changing the levels constantly or riding. There are a variety of ways to tweak the Input levels using a USB mic, sound card and sound editing software. Some USB mics (Blue Yeti) will have a gain control function which adjusts the input level. Other configurations will require you to adjust levels through the soundcard and software programs on the laptop.

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Level Controls

Most digital recorders will have both automatic and manual controls.

Manual Level Controls allow the operator to adjust the input level, recording level or gain manually. Although the operator will have more freedom to be as precise as possible it will generally take more time to set the correct input levels. Some digital recorders (Zoom H2) have gain switch settings of low, medium and high. Generally for interviews the gain should be set on high. You can also adjust the input level in the record standby phase with track left and track right buttons.

Automatic Level Control/Automatic Gain Control (ALC/ALG)are circuits in a recorder that determine an average optimal level. Use of one of these will minimize the risk of clipping but typically not produce as high a quality of recording as manual level control, because they boost quiet moments in the recording up to record level and thus boost background noise. The AGC on the Zoom H2 can be accessed through the Menu and the AGC/Comp function. There you can select AGC2 (Speech) for an interview recording.

A Limiter sets a threshold above which the signal will be gently pushed down in order to prevent clipping. This allows the operator to set optimal levels and minimizes noise while still protecting the recording from clipping. Some digital recorders don’t have limiters (Zoom H1). You can access the Zoom H2 limiter by accessing the digital menu and selecting the AGC/Comp function. Note that limiters are not a substitute for good input levels nor should they be used unless absolutely needed.

A Lo Cut filter is an option designed to remove low frequencies or noise floor in the recording (wind, background hum). Generally this function is found in a switch (Zoom H1) or by accessing the digital menu (Zoom H2). If you’re already recording in a controlled and relatively sound proof environment with little background noise it’s debatable that you’ll need to utilize a lo-cut filter.

 

Microphone technique and placement

The placement of any microphone directly affects the intensity of the recorded signal as well as the signal-to-noise ratio. Theoretical physics or the inverse square law guarantees a loss of approximately 6 dB per doubling of distance from the sound signal or voice (interviewee). Always place the recorder or microphone as close to the interviewee as possible. For this reason, it is recommended to use a mounted condenser microphone or lapel microphone to maintain a close, and constant distance to the source. Ideally a handheld microphone or digital recorder should be placed at a distance of 30 cm or 1 foot away from the mouth of the interviewee. Any further and there is an increased possibility of excessive noise floor being captured in the recording. Excessive moving or handling a microphone or recorder during an interview can cause an excessive amount of peaking. Make sure that the interviewee is not brushing up against the microphone or recorder or unwillingly coming into contact with the table or mic stand.

 

Other Tips

During the pre-interview stage it’s useful to test and monitor levels with your interviewee. Utilize headphones at the beginning of the interview in order to monitor the sound quality of your recording. Levels will not indicate if there is a background noise that you may have not noticed or a loose connection between an external microphone and a recorder. A microphone can greatly exaggerate noise unnoticed by the ear, and can often be rectified by a different, typically closer, microphone placement. Note that the input level and the headphone volume level are not the same thing. Increasing the volume level on your headphones may make you hear the recording better but will not make for a good recording. Make sure that you’re monitoring levels display.

 

Summary

5 main points

1) Have the microphone or recorder about 1 foot or 30 cm away from your subject.

2) Set the automatic or manual input level so that the source (voice) hits between -12 and -6 db.

3) Avoid clipping by having the level to high or noise floor by having the level to low.

4) To avoid unwanted noise or background noise use headphones to monitor the first part of the interview.

5) To make sure you have a good and consistent level during the interview monitor using the input level display on your recording device.