ADPCM vs. MPEG Audio

• The source audio was a digitally generated 1 KHz sinewave, monophonic, 44.1 KHz sample rate, 30 seconds in length, in AIFF format. The amplitude swing was +/- 50%. This produced an -6 dB signal level relative to full 16 bit modulation--to eliminate amplitude clipping in ADPCM or MPEG processing. This -6 dB absolute level was normalized, subsequently termed "0 dB" and used as reference throughout the tests.
• The AIFF source was down sampled to 37.8 KHz using Sound Designer DSP tools. The amplitude was verified to match the source file. I used AudioStack on the Macintosh (the same tool a developer would use) to encode the resampled file to A level, Mono, ADPCM.
• The AIFF source was also encoded into 192 kbits/sec, Mono, MPEG audio, using Audio2MPEG on the Macintosh. The intent was to remove hardware A/D converter and hardware encoding issues. The elementary audio file was multiplexed with a nondescript elementary video file, using PINK. The multiplexed file was processed into an RTF using Master. This stream is designated MPEG #1.
• Another MPEG stream was prepared by encoding a Beta SP tape of color bars and 1 KHz tone. The A/D converter gain at the Delta Vx encoding system was adjusted so that the "clip" light just flickered (-1.5 dB from full 16 bit modulation according Apogee documentation). This is the only "metering" provision on the encoding system. The audio was recorded at 192 Kbits/sec, multiplexed using PINK and RTF'ed by Master. This stream is designated MPEG #2.
• Both the ADPCM and MPEG assets were scripted in MediaMogul 2.2.1 using DV Plugin 2.2.1 and processed with Script2Disc 2.2.1. The default audio attenuation settings were used (no VOLUME or $ command was used to manipulate volume of either asset type).
• A CD-i disc of the build was recorded using CD-IT All 2.5.8. The disc is marked: Level Test (Audio) 5-15-96 MM 2.2.1.
• The CD-i disc was used in all units subject to the test.
• An analog VU meter (with 0.1 dB resolution) was connected to the RIGHT analog audio output of each unit under test.
• The ABSOLUTE level of the signals was not measured and varied somewhat from unit to unit.
• Only the RELATIVE amplitude between the three assets were measured, with the ADPCM level on each respective unit used as 0 dB reference.

Unit               DV Cart        MPEG Level (dB)   MPEG Level (dB)  ADPCM Level (0 dB)
				        (MPEG #1)          (MPEG #2)
210/25            22ER9141/37           0                 +4            0
220/37            22ER9141/00           0                 +4            0
220/57            22ER9141/00           0                 +4.5          0
350 portable      docking unit          0                 +4            0
370 portable      DV01                 -0.5               +3.5          0
550               DV01                  0                 +4            0
605 (carrier)     new                  +0.5               +4            0
605T              new                  +0.5               +4.5          0
PC-I2M            daughter             +7.0               +11           0   (using OptImage CDIPlayback 2.2.1)
MAC-I2M           daughter             +7.0               +11           0   (using CDIPlayback 1.5, ADPCM att=6%)
MAC-I2M           daughter             -0.5               +3.5          0   (using CDIPlayback 1.5, ADPCM att=0%)

• If MediaMogul and/or the DV plug-in were faulty with audio levels or controls at some version, it is has since been fixed and not a factor.
• OptImage has an incomplete compliment of CD-i players. However, given the wide disparity in age and models, the tests provide reasonable confidence. It would appear that the ADPCM and MPEG discrepancy is ASSET PRODUCTION related. ADPCM production tools permit easy control of amplitude, plus the audio industry is very familiar with amplitude matching and calibration procedures in all equipment in the production chain.
• The same CANNOT be said of the MPEG service industry. For Delta V systems, there is little provision for monitoring or calibrating the audio encoding equipment for a known and repeatable amplitude entering the encoding system. Moreover, the encoding systems themselves provide NOTHING for checking amplitudes on the encoded side so that calibration can be achieved
• The 4 dB relative difference from MPEG #1 to MPEG #2 was expected. MPEG #1 was recorded at -6 dB absolute, while MPEG #2 was produced at approximately -1.5 dB absolute (as best we could read the LED). This yields an approximate 4.5 dB difference between the two MPEG assets--confirmed in the tests
• The level discrepancy problem ACROSS PLAYERS, reported last October with MediaMogul 2.2, appears to have been solved by MediaMogul, DV Plugin and Script2Disc versions 2.2.1. The relative levels between the three assets remained essentially identical across units
• The history of the "6%" attenuation setting in CDIPlayback/I2M software is unknown, but apparently misguided. The 0% setting yields much better amplitude matching with controlled assets. Unfortunately, the PC version of OptImage CDIPlayback does not permit adjustment of the ADPCM attenuation--it is fixed at 6%
• Similar tests using the three assets, but under Balboa, yielded essentially identical results of the MediaMogul disc. However, on PC and Mac I2M cards, it seems that Balboa is manipulating the ADPCM attenuation. On the PC, the ADPCM level was only 1 dB below MPEG #1 (compared to 7 dB below with the Mogul disc). On the Mac, the ADPCM level was 7.5 dB below MPEG #1 (same as Mogul disc), but REGARDLESS of CDIPlayback attenuator settings.

• Locate equipment or software that permits monitoring of encoded MPEG audio levels in Real Time or in Post processing
• Locate equipment or software that permits calibration of the A/D converters and audio encoding equipment against an in-line analog or electronic VU meter. This would permit manual level adjustment of the Video Tape machine or other sources, to produce known and repeatable levels. The Delta Vx system just so happens to use an outboard A/D converter with an LED to indicate -1.5 dB absolute level. For Delta Vc and other totally integrated systems, there is no published relationship between volume settings, analog signal levels vs. digital levels, etc. Empirical testing--effective but tedious-- could be used to calibrate the system
• Alternatively, process all audio files using a software audio MPEG encoder. Amplitudes can be controlled/manipulated in standard audio production tools, with seemingly predictable encoding results
• For a given title, both the ADPCM processing chain and the MPEG processing chain MUST be calibrated to the same reference amplitude. Without calibration, the audio levels between ADPCM and MPEG assets will be unpredictable. Most audio experts recommend a "0 dB" reference at an absolute level of -10 or -15dB (absolute 0 dB is full 16 bit modulation) to provide adequate safeguards against clipping. If MPEG and ADPCM processing cannot be mutually calibrated, then at minimum, the calibration references should be published by both so the title developer can make the appropriate adjustments.