MP3 FILES AND NBTV
There is no doubt that CD's are an excellent medium for demonstrating NBTV televisors. That said, it is highly likely that small CD players are going to become less common in the face of competition from MP3 audio and video devices. To the extent that that is true, one might ask if these very compact MP3 players would serve equally well for the display of NBTV images, given what appears to be their more-than-adequate audio quality. Opinion on the NBTV Forum was significantly divided on this question. Some suggested that reasonable results could be obtained while others suggested the project was a waste of time. To answer this question for myself, I undertook a series of tests which can be summarized by the this composite image:
Individual frames were captured using Gary Millard's Big Picture software with no additional image manipulation.
RAW
In order to get the most reliable assessments of image quality, I wanted the best-possible initial image. To that end, I used one of the images from my ROMScanner project. The RAW image represents the output of the ROMScanner applied to the input of the soundcard using the Big Picture for both display and capture. The image is unconditionally stable and the video quality is excellent.
WAV
For this test the putput of the ROMScanner was applied to the input of the soundcard and the signal was recorded in the WAV format using Recall recording software. The WAV file was displayed in three ways:
Using the Big Picture software to display the WAV file directly. This is the WAV picture displayed above.
Driving the soundcard with the output of a high-quality CD player after converting the file to the CD audio format. This was essentially indistinguishable from the result with #1.
Copying the WAV file to a Phillips 2625/37 MP3 player and driving the soundcard with the player output. Given the fact that the line input was being driven, additional amplification was required to develop a reasonable NBTV drive level and the INVERT function had to be employed to cure the out-of-phase output from the player. This resulted in a satisfactory picture, perhaps with very slightly less sync stability than evident in #1 and #2.
It would appear that WAV files prepared directly from an image generator will reproduce the image at a level of quality consistent with the original image source.
MP3F
In order to see what artifacts would be produced by the image compression inherent in the MP3 format, the WAV file was converted to an MP3 file using high-quality sound file conversion software. The MP3 file was then converted back to a WAV file. This file was then displayed in the same way as the original WAV file (above). Sync was reasonably stable when the Big Picture software procesed the file directly and somewhat degraded when using a recorded CD. The image is characterized by many compression-related artifacts that, in the moving image, appear to significantly increase the image noise level. Video quality was about the same when the WAV file was played using the MP3 player, but sync was significantly more erratic.
In summary, conversion of a WAV file to the MP3 format introduces significant compression artifacts. While video quality is reduced, the images are still suitable for display, particularly in the case of moving images. The real question that remains is how do MP3 images fare when played by an MP3 player?
MP3P
The original MP3 audio file was copied to the Phillips 2625/37 player and used to drive the soundcard. Additional amplification was required to get sufficient drive to the line-input to the soundcard and the output had to be invreted as a result of the phase error for this particular player. The results were very poor. Artifacts were far more evident in this run and synchronization was very erratic to the extent that the image was not suitable for viewing. It is evident that MP3 decoding, at least in the case of the player, introduced additional video artifacts and further degraded the sync waveform. Other players might perform more effectively, but this has not been tested.
Mechanical vs. Electronic Displays
It is worth noting that mechanical displays, whose sync is determined by the speed of a Nipkov disk/mirror drum/etc. may fare somewhat better when displaying MP3 files. Electronic display systems react instantly to sync problems, degrading the display instantly. The momentum of a spinning disc may carry the system through such intervals, resulting in less obvious disruption. This assertion has not been tested but may account for varying perceptions with respect to the quality of MP3 NBTV display.
Conclusions
Non-compressed WAV files do not introduce any new display problems that are not already inherent in the imaging source. This statement applied to WAV files converted to CD audio or to WAV files played on a quality MP3 player.
Conversion to the file MP3 introduce obvious compression artifacts, but, if the image was initially of high-quality, it is not necessarily rendered unusable by the MP3 conversion.
The practical problem with using MP3 files with MP3 players is that the decoding software and hardware in such players seem to further degrade both image quality and sync stability. I would suggest that MP3 audio files and players are not suited for routine use.
Some mechanical displays may be more effective in displaying such images, but there would appear to be little reason to bother, given the superior resiults when MP3 players are used with WAV files. WAV files are significantly larger than MP3 files, but one must question is this is a real issue. A 1 GB player (very low-end by current standards) will accomodate about 100 minutes of NBTV video and sound and thus are likely to be suitable for any demonstration.