In the context of audio, “analog” refers to the method of representing a sound wave with voltage fluctuations that are analogous to the pressure fluctuations of the sound wave. Analog fluctuations are infinitely varying rather than the discrete changes at sample time associated with digital recording. Simply put, “digital audio” refers to the representation of sound in digital form.
At the present time, there are four mainstream types of portable digital recorders: solid state, which uses flash memory; hard-disk based, which records to an internal or external hard drive; CD recorders; and direct-to-computer recording which uses an analog-to-digital converter and sends the signal into the computer via firewire or a USB connection. Extensive information on all four types of recording can be found online. My blog Digital Omnium ( contains informational videos about some of the more popular recorders used by members of the oral history community and links to other useful sites. Compact Flash, SD, SDHC, SDHX are the primary media types (at present)
Solid State (Flash Memory) Recorders
- Familiar due to popularity of digital photography
- Flash memory is reusable
- Record at very high quality settings
- Upload at rapid pace
- File-based workflow
- Media is cheap (and getting cheaper)
- Portable, but requires a computer
- Fastest growing portable recorder market
- Large capacity allows for longer recording time and higher recording quality
- Upload/transfer at rapid pace
- Drives are getting cheaper
- Getting more portable
- Still associated with very high-end recording
- Requires a computer and a professional-level audio interface (A/D converter) with good microphone preamps. Internal, manufacturer-supplied sound cards and on-board microphone inputs on a computer are not recommended for interviewing.
- No need to upload until making a backup
- Requires audio-editing software.
Compact Disc (CD-R) Recorders
- CD-Rs are accessible and inexpensive
- CDs are currently dominant in commercial market (but slipping)
- Larger recorders
- Many movable parts
- Must finalize discs before ejecting (usually takes 4 minutes)
Portable recorders possess a variety of input types:
are the highest quality analog inputs. The connection is a “balanced” signal. A balanced connection enables the linking of analog audio devices, including microphones, to a recorder through impedance-balanced cables. Usually associated with professional-level audio equipment, these allow for longer cable lengths and reduce the addition of external noise to the signal. Balanced cables have either XLR or TRS plugs. Professional-level digitization will usually involve balanced outputs on the analog playback device and balanced inputs on the analog-to-digital converter. XLR cables transport a mono signal. Stereo recorders with XLR inputs will contain two XLR inputs. Single Point Stereo microphones will contain a modified version of XLR. In certain cases, this can be a 5-pin connector that splits into two XLR male cables.
A ¼ in. TRS (tip-ring-sleeve) contains the same technology as the XLR connector, but in the form of a ¼ inch connector. The balanced TRS connector carries a mono signal and requires balanced inputs. This allows for greater compatibility with higher-end recorders. A 1/4″ balanced input is fairly rare in portable digital audio recorders.
The1/8 in. mini-plug inputs are typically associated with lower-end recorders. On the connector, one ring signifies a mono jack and two rings signify a stereo jack. The mini-plug’s advantage is portability, but the preamps associated with this input type are usually sub-par. The mini-plug can be temperamental because it does not lock into the microphone input jack and because touching the jack while plugged in can cause static.
The Process of Digital Recording
A microphone converts sound waves into electrical energy which generates an electrical current that is analogous to the frequency and amplitude of the original sound wave.
The digital recorder’s microphone preamps boost the weak electrical signal generating from the microphone
The digital recorder converts the analog signal into digital data (analog-to-digital conversion)
The quality of digital audio is measured by the following parameters:
is the number of samples or “snapshots” taken of the signal and is measured in hertz/second. The higher the sample rate (or the more samples per second) the better the digital representation will be. CD quality equals 44, 100 samples per second or 44.1 KHz and is the minimum recommended sample rate for field recordings.
refers to the number of bits used to represent a single sample. For example, 16-bit is a common sample size. While 8-bit samples take up less memory (and hard disk space), they are inherently noisier than 16-bit or 24-bit samples. The higher the bit depth, the better the recording; however, higher bit depths also lead to larger file sizes.
refers to a measurement of digital audio based on the following equation and is usually expressed in kilobits/second
Bit rate = (bit depth) x (sampling rate) x (number of channels)
Again, CD quality equals 16-bit and should be the minimum bit depth used for field recordings. As flash-based storage media dramatically decreases in cost, 24-bit field recordings are beginning to catch on in the field. These 24-bit recordings are becoming the standard bit depth for archival-quality analog-to-digital conversion– although many repositories and individuals continue to digitize using 16-bit.