Encoding Best Practices, Part 1
Jun 9, 2008 12:00 PM, By Jan Ozer
This month I take a break from our HD production chores to look at streaming-related
issues. In this first installment, I’ll share the results of a survey I recently completed that
details the best practices of high-profile broadcast and corporate streaming producers,
examining options such as codec choice, video resolution, audio and video data rates, and
frame rate. In the next issue, we’ll compare the quality of the big three codecs—H.264,
VC-1, and VP6—using SD, HD, and screencam videos.
About a year ago, I was speaking at a seminar on codec usage and decided to mix a bit of
practice in with my theory. That is, I could talk my heart out about codec quality or the
optimal encoding parameters for streaming, but actual market usage seemed much more
compelling than my own unsupported theory.
So I grabbed a couple of programs that shall remain nameless and started capturing and
analyzing streams of video from well known broadcast and corporate sites. I’ve updated
the statistics twice since then, and I will present my most recent statistics, captured in
May 2008, in this article. For the record, note that I only reported on files that I could
actually download and analyze, which excluded multiple sites.
Nonetheless, the broadcast sites that I reviewed included ABC News, Accuweather,
ABC, BusinessWeek, The Weather Channel, The New York Times, Forbes, Sports
Illustrated, ESPN, CNET, ETonline, The Golf Channel, DL.TV, CBS (show replay), The
Wall Street Journal, CNN, Fortune, MSNBC, and CBS (show preview). On the corporate
side, I looked at PricewaterhouseCoopers, infoDev, PI Worldwide, Deloitte, EMC,
Accenture, E&Y, Chevron, HP (case study), Cisco, Intel, HP (corporate TV), EDS,
Microsoft, and and GE.
I chose the broadcast sites for obvious reasons. On the corporate side, I chose high-profile
organizations that made videos fairly easy to find on their external websites. Note that
many organizations included multiple codecs on their sites. For broadcast sites, I
analyzed the video files most prominent on their home page. For corporate sites, I
analyzed the most recent videos that I could find on their sites.
Overall, I don’t claim that these results are statistically meaningful, but they do illustrate
general trends and best practices.
Encoding Best Practices, Part 1
Jun 9, 2008 12:00 PM, By Jan Ozer
Codec Usage
Let’s start with codec usage which is shown in Table 1.
Clearly, in the broadcast space, Flash had taken over, growing from 50-percent to 88percent usage. In the corporate space, however, Windows Media remains strong, actually
growing in usage since the November 2007 survey.
Some interesting factoids that showed up during my testing. First, the only time that I
saw QuickTime and Real were when they were offered as an alternative to Windows
Media Video files (by Accenture and PI Worldwide). Neither were ever the sole
alternative. Another interesting point was that multiple companies, including Microsoft,
used both Windows Media and Flash, although never as alternatives to each other.
Finally, none of the Flash producers used the H.264 codec, although two broadcast
producers were still using the Sorenson Spark codec. The rest, of course, were using On2
VP6.
Table 2. Output parameters by category.
Click here for a larger image
Resolution and Data Rate
Next, I wanted to examine the resolution and data rate of the video streamed by the
world’s largest organizations, which is critical for several fairly obvious reasons. First, if
CNN is streaming video at 576x324 at a combined data rate of close to 600kbps, this tells
me that most viewers can retrieve and smoothly stream at least this data rate. Second, it
also tells me, assuming my target demographics are similar to CNN, that my target
viewers expect streaming video to look as good as that of CNN.
Table 2 shows these results, which I’ve segmented into three categories for broadcast and
two for corporate with the numbers in parentheses in the Category column showing the
number in each category. I include total pixels in the chart because that’s the best way to
compare the video sizes among the categories. For example, at 201,984, total pixels in the
Broadcast – Aggressive category are almost three times those of the Broadcast –
Conservative category, while those of the Corporate – Conservative category, at 71,314
pixels, are less than half those of the Corporate – Aggressive category.
Table 3. Increase in data rate over 13 months.
Click here for a larger image
Table 3 shows how total data rates (video and audio) have increased over the last 13
months. Clearly, a substantial group of producers assume that their viewers can
comfortably receive more than 600kbps, a dramatic increase since April 2007. Clearly, if
you haven’t re-evaluated the output parameters of the files you’re streaming in the last
year or so, now would be a good time.
Frame Rate
Next, I wanted to determine how the frame rate used by the various entities impacted the
data rate. In theory, one reason to use 15fps as opposed to 30fps would be the ability to
drop the data rate. Certainly this is true in the corporate space, where those producing at
15fps used a data rate 89 percent lower than those producing at 30fps.
In the Broadcast – Midrange group, which was the only broadcast group with a
meaningful sampling of 15ftp and 30fps producers, the difference wasn’t quite that stark.
Those producing at 15fps used an average data rate of 25 percent less than those
producing at 30fps.
Interestingly, however, those producing at 24fps used a lower data rate than 15fps
producers. If you have the option to shoot at 24fps, this may be a technique that allows
you to drop the data rate while retaining both smoothness and visual frame quality. Note,
however, that all producers in the Broadcast – Aggressive group produced at 30fps.
Table 5. Audio data rates for Flash and Windows Media Audio.
Click here for a larger image
Audio Data Rate
The final parameter I reviewed was audio data rate, looking at two specific items. First,
many producers use 128kbps audio just because it’s a default in many templates. Clearly,
if the Broadcast – Midrange group thinks that 66kbps is fine for Flash and 48kbps fine for
Windows Media Audio, it’s probably sufficient for most producers.
I separated these stats out by Flash and Windows Media to highlight the fact that the MP3
codec used in Flash is less efficient than either Windows Media Audio or the AAC audio
codec included in H.264. In fact, some broadcast producers still using Windows Media,
such as Sports Illustrated, broadcast at 32kbps. If you’re planning on switching from
Windows Media to Flash, you'd better count on bumping the same data rate to produce
the same quality.
That's it for this issue. Check back next time for a comparison of H.264, VC-1, and VP6
codecs using SD, HD, and screencam videos.
Encoding Best Practices, Part 2
Jun 23, 2008 12:00 PM, By Jan Ozer
the previous installment, you read about which streaming codecs major broadcasters and
corporate producers were using. Now you’ll see the results of some quality comparisons
that I presented at Streaming Media East in New York last month. I compared the quality
of H.264, Microsoft’s VC-1, and On2’s VP6 codec using both SD and HD test clips,
which I describe further below.
As you may know, H.264 is a standard, and many companies have produced their own
standards-compliant H.264 codecs. For this reason, H.264 quality varies depending upon
who created the codec and the even the encoding tool. As part of my presentation, I
compared the output of three H.264 codecs—MainConcept, Apple, and Dicas—and
found that MainConcept as rendered by Rhozet’s Carbon Coder produced the best
quality, so I used this output to compare against VC-1 and VP6.
Regarding VC-1, note that it’s the same codec as in Microsoft’s more familiar Windows
Media Video 9 codec. It's just a rebranding to reflect SMPTE’s designating VC-1 a
standard—primarily for HD playback devices such as the Samsung Blu-ray BD-P1000
that sits in my living room. For the record, I produced all VC-1 comparisons using
Microsoft’s Expression Encoder 2.
VP6 is On2’s most famous of all Flash codecs, which I produce using On2’s flagship
encoding tool, Flix Pro. There are now two versions of the VP6: VP6-S is recommended
for high-definition production, and VP6-E is best for SD production.
Just before Streaming Media East, On2 announced a VP6 update that supposedly would
deliver 40-percent better quality than the existing codec. Because On2 hadn’t yet
integrated the new codec into Flix Pro, I asked if the company would supply sample
encodes to my parameters using its existing inhouse tools. It did, and I’m showing those
results in the comparisons you’ll see below. However, I'm not scoring the results because
I didn’t have the final codec in hand.
SD Tests
I produced all SD test files at 640x480 at 30fps, with a data rate of 468kbps for video and
32kbps for audio. I produced using two-pass variable-bit-rate encoding (VBR) at the
highest supported profile for H.264. I used encoding VC-1 parameters provided by
Microsoft’s encoding wiz Ben Waggoner. With Flix Pro, I set all quality settings for VP6
at the highest supported levels.
To assess quality, I looked at three factors. To test still-image quality, I captured frames
from each compressed file and compared them side by side. To test motion quality, I
played the files back in realtime and looked for artifacts such as mosquitoes, banding,
jitter, and other artifacts that appear only during realtime playback. Finally, to test
smoothness, I loaded each compressed file into Inlet Technology's Semaphore tool to
detect whether the encoder had dropped any frames to meet the target data rate.
Figure 1. All codecs look the same in this easy-to-compress, low-motion shot.
Click here for a larger image
In each test, I ranked the codecs. Lower scores are better, as summarized in Table 1. As
you can see, H.264 was first, followed by VP6-E, and with VC-1 last.
Let’s start with still quality because that’s easiest to illustrate. As you would expect, all
codecs performed well with easy-to-compress, low-motion footage, and they pretty much
look the same in Figure 1.
Figure 2. In this high-motion sequence, VC-1 is starting to lose detail—and more
importantly, it's dropping frames.
Click here for a larger image
As motion and detail in the video increases, compression becomes more challenging, and
you start to see some differences between the technologies. For example, in Figure 2,
VC-1 starts to look faded and shows less detail in the shirt and in the woman’s face. That
said, at least with these SD clips, the quality differences were quite modest, and probably
wouldn’t be noticed absent side-by-side comparisons.
Note that other encoders produced VC-1 files without dropped frames, and that it may
have been possible to produce files with Expression Encoder 2 that didn’t drop frames—
although this may have changed other results, such as still-image quality or motion
quality. However, I used the encoding parameters supplied by Microsoft, and those are
the results I’m reporting.
Figure 3. While the New VP6 codec didn’t noticeably improve still-image quality, motion
artifacts were much less visible.
Click here for a larger image
Interestingly, I didn’t see much difference between the old VP6 and new VP6 in still
image tests, and neither codec dropped frames. However, the new version of the codec
was noticeably quieter than the existing VP6 during realtime playback.
To illustrate this, I doctored up Figure 3, reducing brightness in the midtones to illustrate
that the background wall as encoded by the updated VP6 codec was smoother than the
wall as encoded by the VP6-E codec that I officially tested, which showed some banding,
a hint of blockiness, and some mottled colors. During realtime playback, this was the
source of much of the distracting noise that lowered VP6-E’s score.
HD Tests
I produced all HD test files at 1280x720 at 30fps, with a data rate of 800kbps for video
and 128kbps for audio. These rates are insanely aggressive, but I started at 1500kbps and
kept reducing the data rate until all the codecs started exhibiting some flaws. I produced
using two-pass variable-bit-rate encoding (VBR) at the highest supported profile for
H.264 and using VC-1 parameters again supplied by Microsoft’s Waggoner. With Flix
Pro, I set all quality settings for VP6 at the highest supported levels. Table 2 shows the
results.
Figure 4. VC-1 exhibits much lower quality than the other contenders in this easy-tocompress clip.
Click here for a larger image
Still-image tests revealed a couple of surprises. First, as shown in Figure 4, VC-1 had
trouble maintaining quality even with low-motion clips such as a talking head. Not only
did VC-1 lose detail, the clip lost color saturation and shows artifacts not present in any
other clip.
Second surprise: the clip compressed with the On2's newly updated VP6 codec showed
substantially greater detail retention than the VP6-S clip that I officially tested, which
looks slightly fuzzy. Both trends are even more evident in the much harder-to-compress
Figure 5, where the VC-1 clip is noticeably more degraded as compared with the other
three frames, and where the "New VP6" clip preserves more detail than the VP6-S clip.
In all comparisons, H.264 exhibited the best still frame quality.
Figure 5. This high-motion test clip makes On2's new version of its VP6 codec look very
promising (but does nothing to dispel concerns about Microsoft's VC-1).
Click here for a larger image
H.264’s advantage over VP6 was reversed during realtime playback tests, where VP6
proved quieter than H.264. Surprisingly, given its stellar performance in SD tests, VC-1
was much noisier during realtime playback than either other codec, probably because of
the still frame artifacts seen in Figures 4 and 5. None of the codecs dropped frames in
these tests, so smoothness scores were equal.
Overall, as you can see in Table 2, VP6 and H.264 tied for HD quality, with VC-1 last.
Note that these results are consistent with past comparisons, where WMV/VC-1 has
ranked last in all three tests that I’ve performed since 2005. Again, this doesn’t mean that
you can’t produce high-quality HD files with VC-1. Rather, it means that you’ll probably
need to encode at a 10-percent to 20-percent higher data rate to produce the same quality
as H.264.
Keeping all this in mind, few producers choose a codec solely based on quality. That
said, it’s obviously always an important consideration and I hope you find this
information useful.