The Case of the Missing 42 Minutes

My job requires me to travel quite often.  This interferes with my ability to watch the few TV shows that I enjoy, namely “House, M.D.” which airs on the FOX network on Monday nights.  To facilitate my ability to watch my shows whenever I want, I rely on a digital video recorder (DVR) type set top box (STB) to record these programs while I’m away.  Awhile back I was on a business trip and eagerly looked forward to watching my favorite show when I returned home.  I ran through the door, dropped my bags, grabbed a bottle of water, jumped over the couch and turned on my home entertainment system to watch the show.  I looked on in horror as the screen menu showed only 18 minutes of “House, M.D.” had recorded…  How could this have happened!  I was on the case immediately.

My first thought was equipment failure; however other shows had recorded normally. I started looking through the entire list of recorded shows and found a few programs that also reported short record times – the DVR did not record the entire show.  This was a mystery that I had to solve.  Being an engineer, I knew I could figure out what was going on.  My first call went to the cable company’s customer service line.  I told them what had happened and they said, “Oh, you probably have a bad DVR.  Bring it in and we’ll give you another one” – and so I did.  I reprogrammed all of my shows (as well as my wife’s) and set about believing the problem was solved.

Several days later, the same scenario occurred again.  This time with a different show… what was causing the exact same failure?  The statistical odds of losing another DVR cable box to the same failure mechanism is astronomically low – unless this was a design defect.  Again, I was on the case.  As before, I checked all the recorded shows and found that several of them had only recorded 15-20 minutes of the program.  All the other shows were intact and would play correctly.  Now I wondered if the programs that did not record properly had something in common… so I began to look more closely.

On initial inspection, none of the shows had anything in common.  They were on different channels which led me to believe that the cable was healthy (no attenuation at a specific frequency to cause data or transport errors).  The only thing that even remotely looked suspicious was the time of recording – all the shows that failed to record properly were recorded during the day time. But “House, M.D.” was the exception… it was on during the night (9:00 PM ET) – however, it was recorded on the original HD DVR.  On the new DVR, all the failed shows were recorded during the day.  What was going on here?  Was the cable system suffering from some failure during day? Had sun spot activity increased causing a loss of the carrier from the satellite or a power failure? But what about my other show, “House, M.D.”? It was not on during the day! So again, I called the customer service line and they said, “Oh, you probably have a bad DVR…”  Once again, I changed out the DVR and reprogrammed all of the shows.

As you can predict, once again the same symptom showed up.  This time it didn’t matter what time of the day a show was aired – it started failing to record properly all the time on almost all the programs.  This was getting worse and there was no solution in sight… until fate gave me a clue. That evening I was standing in front of the rack of equipment where the DVR is located and felt a breeze of warm air coming from the rack. As I thought about the problem it hit me – thermal failure!

I had recently upgraded from an SD or standard definition set-top box without recording capability to the HD DVR version and moved the SD box to the bedroom.  The new HD box was slightly taller than the original – most likely from the additional HD electronics and DVR functions.  Additionally, in comparing the new HD DVR with the older SD box in our bedroom, I noticed the HD DVR drew considerably more power.  I pulled the rack out of the wall and removed the HD DVR box.  On inspection, I noticed that the new design had vent holes on the top of the box where the older SD version did not (it had them on the sides and back).  My theory on the failure was that when I installed the new DVR into the rack space of the original set-top box, I cut off the air circulation through the taller chassis since the vent holes were on the top – not the sides.

For an experiment, I set the HD DVR on top of the rack in open air leaving the equipment out in the hall way (much to the dismay of my family having to walk around it).  Again, I watched to see if the failures continued.  After two days of recording not a single failure.  I called customer service and asked, “Have you had any thermal problems with these new HD DVRs getting hot and not recording?” The person on the other side immediately responded, “Oh yes, these get much hotter than the older ones – you should make sure they have adequate ventilation or they will reset. We get a great deal of returns due to this problem”.  Thus the mystery of the missing 42 minutes of “House, M.D.” was solved.  Here’s how it played out.

The new recorder was installed the week prior to my travels and seemed to run fine.  We had only programmed a few shows so the “recorder” portion (hard drive, associated power sections, etc.) were only being used during the late night when it was cool.  We started adding more shows – many of which aired at the same time causing both tuners to be active. The increase of active circuitry generated additional heat in the system which caused the temperature in the box to rise. If the DVR had been recording for several hours non-stop, the internal temperature would rise above the maximum operating conditions and cause the unit to fail. On failure, it would reset and stop recording, cool down and operate normally again.

The replacement boxes showed the same symptom, only at different failure levels – the last box being the most sensitive.  In a way, the more sensitive unit made finding the problem much easier.  The day time failures suddenly made sense as well – the air conditioning during the day is set up to 82 degrees F (28 degrees C) which is much warmer than during the evening when we are home – so that particular box was failing only during the day.

So, what is the moral of this story? I’m glad you asked.  In a residential (or commercial) space, having thousands of watts available from a wall socket doesn’t mean it’s acceptable to draw as much power as you want (or can).  Power that goes into electronics comes out as heat.  Heat flows from higher to lower temperatures (a temperature differential) at a rate defined by the thermal impedance of the system.  If heat is continuously added to a closed system, temperatures will rise until a temperature differential is achieved that allows the heat to flow. In the case of my new HD DVR, my original rack space was fine for an SD unit, but inadequate for the HD DVR’s increased power consumption. 

Designers should always strive to lower their power consumption since there is no guarantee that proper ventilation will be available.  In the case of the new HD DVR, designers solved the thermal issues by simply drilling more holes in the box which in my case caused 3 consecutive failures – and many returns for the cable company from other customers.  A lower power design would have slipped right back into the rack and never missed a single episode of “House, M.D.” 

Got a similar story?  Drop me an email or comment here on the blog.  I’d love to hear from you!  Till next time…