Digital Audio/Video and Communication Rates

By Amir Majidimehr

When your favorite cable company offers you "8 Mbits/sec" Internet service, do you know how fast that really is? How fast is that compared to CD audio? How about high-definition video broadcast in US? If you don't know the answers to these questions (and most people don't), this is the article for you. Without further introduction, here are the most common standards you are likely to run into.

Audio/Video formats/standards:

CD audio: 1.4 Mbits/sec (2 channels * 44,100 samples/sec * 16 bits each)

Lossless Compressed CD audio (WMA Lossless, Apple Lossless, FLAC, etc): Variable: Peak 1.4 Mbits/sec, average 0.5 to 0.7 Mbits/sec. Average rate determines the capacity requirements for storage. Peak determines how fast of a channel (e.g. home network) you need to have for the stream to play without interruption.

High Resolution audio: Fixed bit rate proportional to audio sample rate and resolution. 2-channel, 96 KHz, 24-bit = 4.6 Mbits/sec. Double that for 192 KHz audio.

Lossless High Res audio (MLP, WMA Lossless, Monkey, FLAC, Dolby TrueHD): Peak: the same as uncompressed. Average: divide the peak by 2 to 2.5.

Typical Internet video (e.g. YouTube) data rate: 0.3 to 0.4 Mbit/sec (320x240 resolution or quarter of DVD)

DVD: Peak 10 Mbit/sec (“1X drive”), average 4 to 5 Mbits/sec

US digital TV standard: Channel capacity 19.2 Mbits/sec. Actual encoding rate varies based on how many channels are stuffed into that one channel (convention is one HD channel plus one more SD channels). Typical rates are probably in 12 to 14 Mbits/sec for HD channel. Encoding is at “constant bit rate” meaning the data rate is fixed over a small period of time.

Satellite companies (and soon most cable systems ) use “Stat Mux” (statistical multiplexing) where channel capacity is variable and can be pushed up and down as needed. For example, pay channel movie may get more bitrate when it needs it during a high motion sequence, coming at the cost of reduced bits allocated to a channel like “golf.”

Blu-ray Disc: Peak 48 Mbits/sec for *all* audio and video tracks. Video peak = 40 Mbits/sec. Average probably in 20 to 25 Mbits/sec (title dependent). Again, peak rate determines streaming requirement and average is what you need to store the movie.

Communication channels:

Ethernet: Available in 10, 100 and 1000 Mbits/sec. 10 Mbits/sec is ancient and not in much use today. 100 Mbits/sec is most common. 1000 Mbits also called “gigabit” Ethernet (1000 mega = giga) or "Gigi" is also becoming popular. These are theoretical rates and the equipment may not be capable of keeping up with it. Most devices sporting a 100 Mbits/sec Ethernet port can drive it to near saturation which is around 90 to 95% of capacity of 90 to 95 Mbits/sec (i.e. two Blu-ray streams playing simultaneous). Gigabit Ethernet throughput is harder to predict because it could be throttled by other components in the system but is likely at least 5 to 6 times higher than 100-Mbit Ethernet.

Wireless: This comes in different specs from 11 Mbits/sec for the older “b” network to 54 Mbits/sec for “g” and 500+ Mbits/sec for the “n” version. Alas, due to complexity and variability of wireless transmissions, actual data rates are unpredictable and often far lower than these maximums. Worse yet, they can change on a moment's notice as anyone who has gotten a garbled cell phone call can testify. As a result, even the 802.11n may be unsuitable to stream a single Blu-ray movie let alone multiple without stuttering (starvation for data). Wireless streaming works fine for shared data, and streaming of audio however.

Broadband Internet connection: No fixed standard here but taking Comcast as the largest carrier out there, nominal specification is 15 Mbits/sec. In Seattle area they offer the "BOOST" feature which has a claimed data rate of 30 Mbits/sec. Additional tiers up to 50 Mbits/sec are also available. Actual throughput varies wildly and are probably 3 to 5 times lower during busy times (and sometimes far worse). Note that your home wireless connection can become a bottleneck at these higher offered speeds if you are for example using a wireless network.

Wireless cellular Internet connection: There are a few standards there. Verizon EVDO comes in two flavors with the older standard peaking at 2.4 Mbits/sec and the "Revision A" at 3.1 Mbits/sec. AT&T's newer network (NOT EDGE) use HSPA 7 which as you can guess, has a peak rate of 7.2 Mbits/sec. In reality, you can't even remotely come close to achieving these rates due to the fact everyone shares the same channel to the local transponder and that wireless signal quality can go up and down drastically. Actual throughputs as high as 0.4 Mbits/sec are very possible on Verizon network but it can also go down to 0.1 Mbits/sec. These data rates are insufficient for reliable streaming of video motivating these companies to invest in so called "4G" and WiMax networks.

USB 2.0: while not a network connection per-se, it is the connection used on PCs to connect for example hard disks which may have A/V data on them or be used to connect a DAC (digital to audio converter). USB 2.0 maximum rate is 400 Mbits/sec with actual rates probably 80% of that. Plenty fast for both high definition audio and video.

USB 3.0: The up and coming revision of USB. Peak rate: 4,800 Mbits/sec. Actual data rate is probably 50 to 60% of that or roughly 10X of USB 2.0. Of note, it has more electrical power so that you can run hungrier devices without a power supply than current USB 2.0. USB 3.0 is just starting to ship as of January 2010.

SATA: Serial “ATA” interface used to connect storage devices. Current 2.0 revision has peak rate of 3,000 Mbits/sec. Actual rate is 1000 to 2,500 Mbits/sec.

Storage devices:

CD-ROM: Does anyone buy these anymore? Probably not but let’s describe the common nomenclature used to spec DVD drives reading CDs. “1X” is one times the speed of audio CD or 1.4 Mbits/sec. So a drive rated at 20X, is able to read the CD at 20 times real-time. In reality, it can only do this at one end of the media and is slower at the other end. Actual rate across the whole surface of the disc is probably half of this.

DVD: As implied in the data rate of DVD movie, achieved data rate of the drive is 10 Mbits/sec which is also called “1X.” Consumer devices use 1X drives but computer drivers spin the media much faster as specified in their “X” rating. As with CDs, they cheat and actual rate is probably 50 to 60% of the stated value. A DVD drive rated at 4X, will rip (copy) a 90 minute movie in about 30-40 minutes.

Hard Disk: Manufactures usually cheat and spec the interface data rate which is the same as SATA (i.e. 3,000 Mbits/sec). But this is in the rare cases where the data exists in the tiny memory onboard the drive. The specification you are interested in is how fast the drive can transfer data stored on its storage platters (which is usually buried in the specs of the drive on the web site of the manufacture). Actual data rate varies by drive type and manufacturer. For example, a Western Digital 640 Gigabyte “blue” drive is able to sustain 1,000 Mbits/sec. Yes, a single drive can saturate a gigabit network, and stream up to 21 Blu-ray movies simultaneously! And heaven knows how many audio streams. The icing on the cake is that this is not nearly the fastest drive you can buy (although faster drives are smaller in size).

The above means that you don't need to bother with "RAID" drives and such to get the throughput you need for audio or video in your home. Single drives work just fine.

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