Instruction: Answer the given question by extracting information from the supporting document.

Input:
Question: what is the update port on the back.
Document: The look of the unit is excellent. It's very handsome and has the appearance of quality. The front panel has inputs for video and the audio associated with a video source. I'm going to try to use this for my iphone. I'll try to post an update when I figure it out. It has sub-woofer outputs on the back. I'll be using those later.Would I recommend this to others?
At this time I am thinking it may be my issue, with how I wired it or something. Actually, I am going to read through the other reviews again to see if anyone else is experiencing the same problem. I will try to update this later if I find out anything more. Happy shopping.
Make sure that you have speakers with adequate low-end bass to compensate for the lack of a loudness feature. A powered sub-woofer with controls will help.Update 16 January 2011Yep, the connections on the front can be used for an ipod/iphone. In fact it comes with a cable. This will be just fine until I can get a dock.
Oh well...back to spinning more vinyl!
I'm so glad that I did! There aren't any others that compare. Suround sound systems BEWARE. This is real music! Thanks to Amazon and HK, I'm back in business.
I tried an Onkyo 607 (see my review) and found it to very good for HT but not very good for music. I had to send it back for a static defect anyway.
Although it costs a lot more than this discounted model, it delivers more power and has many more useable features on the receiver and remote. Or maybe I'll go back to Pioneer.
The Bass and Treble are a easily concealed as you have to press them to get them to pop out, adjust then pop back in. Matching covers are supplied to cover the tone and RCA outputs on the front giving it a smooth clean look.
bought this receiver to play back pandora from a squeezebox player in the master bedroom. Coupled this receiver with two Def. Tech. ProMonitor 1000 bookshelf speakers. The sound is wonderful. Purchased back in August of 2009.
Replaced a Sherwood 100 W unit. The HK sounds much better with the same speakers. Controls are easy to use. Nice quality binding posts,The switched outlets on the back give us options for lights or other inputs.
Answer: That is for certified technicians to use if you take it in for service. From the manual: Do not make any connections to this port. It is reserved for use by authorized service personnel in the event that a software update becomes available.source: http://www.harmankardon.com/images/media/HK3390_OM_EN.pdf