Suggested Road Map for Best Practices Document for Analog-to-Digital Conversion
January 30, 2004
George Massenburg, a recording engineer and producer, and AES representative to the NRPB, presented this document to the participants on the second day of their meeting. Its purpose was to facilitate conversation about analog-to-digital conversion. The document sparked lively debate about such issues as the preferred sampling rate (e.g., for reformatting of audiocassettes) and evaluation criteria. Participants subsequently recommended that a separate meeting be focused solely on the topic of conversion. Mr. Massenburg’s document is produced here as a record of the meeting.
Objective: To capture complex analog signals with as much transparency to the original as practical.
1. Converter hardware criteria
1.1 Technical evaluation issues
1.1.1 Bit depth
Recommend: 24 bit
1.1.2 Sample rate
Recommend: 96 kHz or better
1.1.4 Stability of clock reference (perhaps external)
Recommend: 100ps or better RMS jitter
1.1.5 Type and quality of anti-aliasing filter (mandate optimizing brick wall filter to track sample rate)
- level (weighting)
Recommend: 110dBFS A-weighted or better
- quality and nature of artifacts
Refer to FFT evaluation of performance, observe relatively few spikes
1.1.7 Dither (word-length reduction) methodology (if applicable for 16-bit archives)
1.2 Perceptive evaluation issues and protocols
1.2.1 Establish testing protocols
- choose protocol, preferably utilizing two or more methods; from simple (alternate D/A/D process with original analog source) through multidimensional scaling, A-B-X, et al.
- establish conditions, including suggested standardized monitoring and listening rooms
- establish trusted reference D/A converter to check complete conversion process
- verify level calibration to ±0.05 dB max, in particular the A-B path and the SOURCE (analog original) vs. D/A converter path
1.2.2 Establish source reference materials
- consider breadth of “quality” (especially low-quality end) of different ingested materials
- establish categories
1.2.3 Isolate evaluation criteria
- perceived resolution: How well are very low-level sounds articulated, especially low-level, high-frequency percussives and transients?
- spectral balance: perceived “flatness” and “neutrality,” lack of harshness
- image stability: In stereo and multichannel, is the spatial image laterally stable?
- organization of report database
2.1 Baseline procedures
2.1.1 Standardize subjective evaluation criteria
2.1.2 Develop conversion objectives
- budget contingencies
- estimate archive lifetime
- ongoing costs versus quality
- storage type efficiency/reliability/commonality
Observe that the costs of storage technologies are constantly shifting; determine possible interchangeability issues with other archives.
2.1.3 Establish evaluator group constituency
2.1.4 Establish consistency criteria across evaluators/technicians
2.1.5 Establish technical specifications (e.g., dynamic range, noise, flatness) for equipment and specifications for monitoring, including monitor speakers and room acoustics
2.1.6 Establish criteria (suggested is a “transfer confidence index”) to determine whether and when original materials must be retained for future reconversion
2.2 Archival processing-transfer
2.2.1 Specify conversion parameters within latitude of conversion options, if scaling of methodologies is determined to be acceptable
Examples: Recommend limiting single-channel conversion and single-channel files for purely monophonic sources (requiring more expert evaluation); recommend a deprecated bandwidth and resolution for limited-quality originals (for instance, old 78s and cylinders)
2.2.2 Determine the “transfer confidence” index
To what degree do I think that this conversion represents the best (i.e., maximally transparent, to a described, agreed-upon standard)?
2.2.3 Determine whether the specific original physical archive element is so deteriorated that improvements in conversion would provide little or no further advantage to existing archive materials
2.2.4 Establish whether and when original materials can be disposed of with confidence
- establish a redundancy protocol for a given digital archive and for the migration of the digital archive media as well as the underlying digital archive technology
- verify effectiveness of the integrity of the migration protocols including background archive soft error checking
2.3 Ongoing review
2.3.1 Calibrate and do technical reevaluation of equipment
2.3.2 Review and oversee methodologies; track technology
2.3.3 Correlate the database’s “transfer confidence” index with possible improvements in technology