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First published March 2012.
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Speed of USB 2.0 drives: Suitability for video recording

Many set-top boxes now come with the ability to use an external USB drive to make recordings. The expectation is that an external USB hard drive will be used to make recordings because these have sufficient capacity for many hours of recording but there is no reason in principle that a flash drive (thumb drive; USB stick) cannot be pressed into service for a one-off recording or even a few recordings.

During 2011, set top box testing revealed that some boxes were sensitive to the model of USB drive that was attached as a recording drive. In particular, the Dish TV T1020 DVB-T receiver was intolerant of several ordinary flash drives, although it did record satisfactorily using a WD 500GB HDD. This test started as one to assess suitability of USB drives for recording from set-top boxes, but evolved into a straight comparison of read and write speeds. The read/write speeds of flash drives and particularly their write speed varies considerably.

Testing procedure

We used a free utility called CrystalDiskMark 3.0 from Crystal Dew World to determine the read and write speeds of ten readily available drives. Most of these are cheap USB sticks but included for comparison are hard disk drives and a couple of memory cards via a USB card reader. The test was done on a USB 2 port of a pc with Intel Core i7 870 processor/ 8GB G-Skill RAM and running Windows 7 professional/64 bit.

The software does three separate read and write tests on each drive. The first was set to read and write a continuous large (100MB) file to the device; the second test is a random 512kB file test and the third is a small 4kB file random test. Results of all these speed tests are shown below in no particular order.

speed test result chart

There are several points of interest from the above results:

Suitability for recording

It is expected that for video recording purposes, the 100MB sequential tests will be most relevant since the data will be streamed to and from the memory device in continuous fashion. Reading from the disks does not seem to be a problem. However, we saw the difficulties with the T1020 receiver last year when writing (recording) a TV programme. From the chart, the best flash drive for writing a large file to is No.3; the 16GB Toshiba which can take a write at 12.8MB/s. This was bought earlier this year from Warehouse Stationery for NZ$24.95. The worst of the flash drives at writing the 100MB chunk is No.6; the generic 256MB stick which manages to write at only 3.2MB/s. This is an oldie and of insufficient capacity for any realistic video recording, but illustrates the large variation of writing speeds from these devices. The xD card (test 7) is even slower, but I have no means to separate the performance of the card from that of the USB card reader. The Sandisk Compact flash card is virtually as fast as the two Western Digital hard drives, despite the fact the CF card is interfaced via a Vantec card reader. Since the CF card is advertised as being 30MB/s capable and actually is, I can conclude that the Vantec card reader itself causes no significant restriction of speed. The Sandisk CF card and these two WD hard drives are easily fast enough to record a digital programme stream from a Freeview receiver, but what can we say about the flash drives?

Freeview data rates

Video stream data rates of broadcast digital TV are variable. They are encoded that way. Static scenes, or slowly changing scenes have a low data rate. Rapidly changing scenes, especially those with lots of detail require a much higher data rate. Freeview in New Zealand operates two transmission platforms. The satellite platform provides standard definition programming only, using MPEG-2. The terrestrial platform (UHF) has both SD and HD programming, but uses the more efficient H.264 encoding format, which is also known as AVC or MPEG-4 Part 10.

On the UHF platform, HD programmes typically average 4-5Mbits/second with a maximum of 9Mbits/sec. SD programmes average about 2Mbits/second with a maximum of some 3.5Mbits/second. For the satellite service, I estimate the data rate as averaging around 4Mbits/second. To try and confirm the data rate from the satellite service, I recorded short segments to each of the USB drives from an Openbox S10 receiver. From seven separate recordings ranging from 1 minute long to 16 minutes long, the file size recorded on the disk means that the data rate ranged from 0.33MB/sec to 0.59MB/sec, thus corresponding to a low of 2.64Mbits/sec and a high of 4.72Mbits/sec. It is possible that over shorter periods, a greater range will be seen, but this is in the ballpark of what we expect. None of the flash drives showed any difficulties recording from the OpenBox receiver.

From these recordings, it would appear that even for HD programming, a write speed of 9Mbits/second, or around 1.2MB/second would be all that is required. On the surface of it, all the flash drives would seem to be capable of writing fast enough to cope with recording an HD data stream.

Difficulties with the Dish TV T1020

When we tested several set top boxes last year, it was only the T1020 terrestrial model that gave trouble when attempting to record to a flash drive. The symptom was that the picture would pixelate and break up for a few seconds. This breakup was actually recorded on the file. The receiver showed this symptom with three different flash drives, but did not have a problem when recording to a WD hard drive. The Dish TV handbook actually states that some flash drives may not be suitable. Since all the other receivers tested did not exhibit the problem, we concluded that the T1020 receiver probably has insufficient data buffering to cope with (especially) HD data rates. The current test of flash drive write speeds appears to find that all those tested, and which were used to test the set-top boxes 'should' be fast enough. The limitation of this speed test is that it is rather coarse. That is, the results are at first an average over a period and secondly, the best of three runs is presented. It is still likely that the peak instantaneous video rates exceeded the write capability of the drives. High peak rates might occur for only a fraction of a second and which most likely corrupted the T1020 runtime routines. A larger data buffer would mean that the system only need be fast enough to cope with an average long term data rate. To confirm this would require better diagnostic tools and a more extensive test.

How many minutes of recording per GB capacity?

The recordings made on the OpenBox S10 from the Freeview satellite service gave the figures below:

The above are for MPEG2 encoding from the satellite platform. The Freeview terrestrial platform using H.264 encoding will result in recording time for SD programmes of around 45 minutes (or longer) per GB. For HD programmes, you could expect to get no more than 25 minutes per GB of capacity. So, using a 16GB stick you could record about 6 hours of HD programmes or up to 12 hours of SD.

Note: To avoid confusion, I have used GB/s to mean gigabytes per second but have written the longhand Mbits/sec. 8 bits to a byte.

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