error control experiment comprised of five measurements, with the
measurement of no error
control being the control case of the experiment. The metric for measuring the
quality of the video is the comparison of the signal-to-noise (SNR) ratio of the resulting
video to the original video. The measurements were no error control (the control
case), HEC, and 4-, 8-, and 16-bit symbol RS FEC. Figures 2 through 4 show the
results of the experiments for each error control method with error rate as a function of
the SNR. Higher SNR ratio means better image quality. A SNR
ratio of 47 is a perfect duplicate of the original video, and an SNR
of 0 indicates a premature termination
of MPEG player due to decoder error.
Error Rate and SNR Using RS FEC.
the quality of the video decreases as error rate and/or burst error
size increases. It is observed from the
experiment that an error rate less than or equal to 10-6,
with small burst size (less than 10 bits), yields nearly perfect image
quality. Visually, errors are not
perceived. However, this is subjective. With error rates greater
video quality starts to degrade severely without error control. Especially
with a burst error size larger than or equal to 5-bit, the MPEG decoder
encounters errors and thereby
terminates, causing the MPEG player to quit prematurely. Furthermore,
the data does not show that with an error rate greater than 10-6 and
a small burst error size, the video will not be played with the correct
frame rate without error control. It pauses every now and then
and usually skips frames. Thus error control must be provided to guaranteed
quality of service.
The RS FEC error control method provided superb error control. It
corrected all errors under all experimented conditions, except the 4-bit
symbol RS FEC when the burst
error size was larger than 10 bits. The
quality degraded slightly in this case. It was found that a larger
code symbol size of RS code is
favored to handle larger burst size error; a smaller code symbol size
is favored to handle scattered bit errors. Furthermore, by increasing
the number of redundancies appended, the
number of burst errors it can handle also increases. However, this is
a tradeoff, which causes a larger overhead processing time in the
encoding and decoding process.
Despite the superb error correction ability of RS FEC, it imposed a large overhead
redundancy and overhead processing time. The redundancies imposed by the 4-, 8-, and
16-bit symbol RS FEC were 25, 14.4, and 32 percent, respectively. The imposed
redundancy by the 16-bit symbol RS FEC was unusually large because its block size is
relatively large compared to the packet size; therefore extra padding was required. Large
overhead redundancy and overhead processing time resulted in lower throughput and higher
latency delay. In either case, the number of concurrent clients a single server can
handle decreased. The performance of the system was decreased to
maintain relatively perfect image quality.
Error Rate and SNR using HEC.
The HEC error control method provided
a relatively perfect image quality with error rate less than or equal to 10-6,
an acceptable image quality with error rate of 10-5, and ensured the continuing
playing of the video with error rate of 10-4. The overhead redundancy was
very small compared to the RS FEC error control method, 1.34 percent. However, the
overhead processing time is about the same as with RS FEC because of the parsing and the
rearrangement of sequence headers. The software implementations for both proposed error
control methods are not practical because of the large overhead processing time imposed.
Thus hardware implementation is recommended for such tasks.