Genealogy: Audio recordings

Genealogy: Audio recordingsBy Barry J. Ewell

Audio recording began in 1877 with tin foil on a cylinder, which was commercialized with the introduction of the wax cylinder recorder by Thomas Edison. Since that time audio recording devices have gone through a series of technological advances. It is possible to digitally reformat any of these commonly encountered recording media which are summarized below. Audio quality will vary based on the type of device, quality of the original recording, and the software employed.  Modern computer-based audio editing software has made it possible to “clean up” the audio in most cases within the limits mentioned above.

Audio Recording Formats

  • Format: Wax Cylinder Records
    • Description: About 2 – 4 minutes, wax or wax compound
    • Years in Use: 1877 – 1929
  • Format: Recordable Disk Records (Direct or Acetate Disks)
    • Description: 7, 12 or 16″ recorded at 33 1/3 or 78— generally vinyl on a paper, glass or metal base
    • Years in Use: 1929 – 1960s
  • Format: Recording Wire
    • Description: Spooled wire, usually in 15 – 30 minute lengths, one direction only
    • Years in Use: c1945 – 1955
  • Format: Open Reel Recording Tape
    • Description: 1/4″ – 2 inch, 3 – 10 1/2″ reels, 15/16- 30 IPS (inches per second) speeds
    • Years in Use: c1945 – Present
  • Format: Compact Cassette
    • Description: 1/8” tape in hard case. 1 7/8 IPS format (also 15/16 & 3 ¾)
    • Years in Use: 1965 – Present
  • Format:Microcassette / Mini-cassette
    • Description: Very small 2 – 4 cm cassette tapes
    • Years in Use: 1977 – Present
  • Format: Digital disk,MP3 (and other digital recorders)
    • Description: Audio recorded directly in digital files to optical disks or internal hard drives
    • Years in Use: 1984 – Present

Sample rate and bit depth
Sample rate defines the number of samples that are recorded per second. It is measured in Hertz (cycles per second) or Kilohertz (thousand cycles per second). The following table describes four common benchmarks for audio quality. These offer a gradual improvement of quality at the expense of file size.

Description of the Various Sample Frequencies Available

  • Samples per Second:8kHz
    • Description: Telephone quality
  • Samples per Second:11kHz
    • Description: At 8 bits, mono produces passable voice at a reasonable size
  • Samples per Second:22kHz
    • Description: 22k, half of the CD sampling rate—at 8 bits, mono, good for a mix of speech and music
  • Samples per Second:  44.1kHz
    • Description: Standard audio CD sampling rate—a   standard for 16-bit linear signed mono and stereo file formats

Bit rate indicates the amount of audio data being transferred at a given time. The bit rate can be recorded in two ways – variable or constant. A variable bit rate creates smaller files by removing inaudible sound. It is therefore suited to internet distribution in which bandwidth is a consideration. A constant bit rate, in comparison, records audio data at a set rate irrespective of the content. This produces a replica of an analogue recording, even reproducing potentially unnecessary sounds. As a result, file size is significantly larger than those encoded with variable bit rates.

Indication of Audio Quality Expected with Different Bit Rates

  • Bit Rate: 1411
    • Quality: CD quality
    • MB/min: 10.584
  • Bit Rate: 192
    • Quality: Good CD quality
    • MB/min: 1.44
  • Bit Rate: 128
    • Quality: Near CD quality
    • MB/min: 0.96
  • Bit Rate: 112
    • Quality: Near CD quality
    • MB/min: 0.84
  • Bit Rate: 64
    • Quality: FM quality
    • MB/min: 0.48
  • Bit Rate: 32
    • Quality: AM quality
    • MB/min: 0.24
  • Bit Rate: 16
    • Quality: Short-wave quality
    • MB/min: 0.12

The majority of audio formats use lossy compression to reduce file size by removing superfluous audio data. Master audio files should ideally be stored in a lossless format to preserve all audio data.

Common Digital Audio Formats

  • Format: MPEG Audio Layer III (MP3)
    • Compression: Lossy
    • Streaming Support: Yes
    • Bit Rate: Variable
    • Popularity: Common on all platforms
  • Format: Mp3 PRO (MP3)
    • Compression: Lossy
    • Streaming Support: Yes
    • Bit Rate: Variable
    • Popularity: Limited support
  • Format:OggVorbis (OGG)
    • Compression: Lossy
    • Streaming Support: Yes
    • Bit Rate: Variable
    • Popularity: Limited support
  • Format: RealAudio (RA)
    • Compression: Lossy
    • Streaming Support: Yes
    • Bit Rate: Variable
    • Popularity: Popular for streaming
  • Format: Microsoft Wave (WAV)
    • Compression: Lossless
    • Streaming Support: Yes
    • Bit Rate: Constant
    • Popularity: Primarily for Windows
  • Format: Windows Media (WMA)
    • Compression: Lossy
    • Streaming Support: Yes
    • Bit Rate: Variable
    • Popularity: Primarily for Windows

Conversion between digital audio formats can be complex. If you are producing audio content for internet distribution, a lossless-to-lossy (e.g. WAV to MP3) conversion will significantly reduce bandwidth usage. Only lossless-to-lossy conversion is advised. The conversion process of lossy-to-lossy will further degrade audio quality by removing additional data, producing unexpected results.

Pros and cons of sample rates and bit depths

  • Sample rates:  As you digitally record your audio there are two acceptable choices: 96 kHz and 44.1 kHz. The 96 kHz sample rate is accepted as the standard for archive preservation.  If you are not limited by your computer processing resources, then seriously consider using 96 kHz, otherwise go with 44.1 kHz.
  • Bit depth:  The selection of a bit depth is the second critical factor in determining the audio fidelity of the digitally reformatted audio. Adhering to the professional standard 24 bit depth is recommended.

Comparison of Pros and Cons of Sample Rates and Bit Depths Minimum

  • Sample Rate: 44.1 kHz
    • Bit Depth: 16 bit
    • Pros:
      • No file format conversion needed for Audio CD.
      • Maximizes storage space.
      • Appropriate for lower quality source files.
      • Lowest level of processing time.
      • Ubiquitous home audio standard.
      • International standard for Compact Disc (Red Book Standard).
    • Cons:
      • Lowest frequency range acceptable.
      • May not provide sufficient quality for future formats.
      • May have limitations for publication or broadcast, and migration to future digital formats.
      • Limits ability to enhance source file for delivery.
  • Sample Rate: 44.1 kHz
    • Bit Depth: 24 bit
    • Pros:
      • More accurately reproduces sound of source material.
      • Increased capability to enhance source file for delivery.
      • Increased dynamic range.
      • Acceptable for publication and broadcast.
      • Reflects current professional audio standards.
  • Sample Rate: 96 kHz
    • Bit Depth: 24 bit
    • Pros:
      • Standard for DVD/HD Audio.
      • Increased frequency range.
      • More accurately reproduces sound of high frequency, high quality source material such as musical recordings.
      • Increased potential for enhancement of source file for delivery.
      • More potential for future applications.
      • Potential recommended benchmark for future.
      • Highest recommended current quality.
      • Rapidly growing acceptance.
      • Reflects emerging professional audio standards.
    • Cons:
      • Increased storage space.
      • Increased processing time.
      • No perceptible improvement in sound quality for some source files.
      • Requires conversion to 16-bit and 44.1kHz for delivery on Red Book Audio CD.
      • May require frequency compression for delivery.

File Size in Megabytes
This table represents the file sizes for 1 hour of digitized audio at different sample rates and bit resolutions.

  • Sample Rate kHz: 44.1 kHz
    • Bit Resolution: 16
    • No. of Channels: 2 (stereo)
    • File Size Megabytes: 591 mb
    • File Size Gigabytes: 0.59 gb
  • Sample Rate kHz: 44.1 kHz
    • Bit Resolution: 16
    • No. of Channels: 1 (mono)
    • File Size Megabytes: 296 mb
    • File Size Gigabytes: 0.30 gb
  • Sample Rate kHz: 44.1 kHz
    • Bit Resolution: 24
    • No. of Channels: 2 (stereo)
    • File Size Megabytes: 887 mb
    • File Size Gigabytes: 0.87 gb
  • Sample Rate 44.1 kHz
    • Bit Resolution: kHz: 24
    • No. of Channels: 1 (mono)
    • File Size Megabytes: 444 mb
    • File Size Gigabytes: 0.44 gb
  • Sample Rate kHz: 96 kHz
    • Bit Resolution: 24
    • No. of Channels: 2 (stereo)
    • File Size Megabytes: 1931 mb
    • File Size Gigabytes: 1.93 gb
  • Sample Rate kHz: 96 kHz
    • Bit Resolution: 24
    • No. of Channels: 1 (mono)
    • File Size Megabytes: 966 mb
    • File Size Gigabytes: 0.97 gb

Audio file types
As you convert to digital audio files, remember there are many file types available.  As you consider file types, you are looking to choose those that can be read by a variety of software.  File types that compress the audio file should be avoided for preservation masters.  Both of the following file types are uncompressed and acceptable for long-term file storage.

  • WAV file type was developed by Microsoft, it is in widespread use, and is readable by virtually all audio software programs. WAV file type has become a standard and is recommended. In addition, the WAV file type is also available in a professional flavor, broadcast WAV (BWF), which has the capability to store metadata in the file header. Although not all audio software programs are currently capable of reading or writing to the metadata header, the BWF format is emerging as the WAV file type of preference for archival audio projects.
  • AIF file type was developed by Apple, Inc., and is also in widespread use.

With the emergence of compact hand-held digital recording devices, a number of other formats have become very popular. Although compressed file types are ideal for Web applications, none are recommended for long-term storage.

  • MP3 format has become the file type of choice for many applications requiring downloads or uploads to the Web. This format has the advantage of being highly compressed for electronic transfer, but also has the disadvantages of compressed formats.
  • Windows Media, Real Audio, and Quicktime: These programs are used for streaming downloads. Caution should be taken to avoid the proprietary formats of many of the hand-held media players.