PCM or Bitstream

I have a dvd player that has the option to play bitstream or pcm.I am wondering which is what I am supposed to use.I have a/b ed them with several recordings and I can not seem to pic up on a difference in sound.What gives with this?
That player of yours must have two sets of DAC's inside it (on two chipsets, or both on one? I don't know). If the sound is not different enough to detect to your ears in your system, then I wouldn't worry about the choice. Another way to look at the situation would be to assume that quite possibly other, presumably shared aspects of the player, such as the transport mechanism, power supply, analog output stage, and output filter, may determine its characteristic sound to a larger degree than the method of conversion chosen.
Pcm uses your players Dac..Bitstream uses your processor's Dac. Most (not all) pre/pro's have the better Dac so you should hear some kind of difference. Do you have a good digital cable? Maybe both Dac's are very close in quality.

I suspect that the reason it seems to make no difference is that there is none in your setup.

If you are using the digital output to the preamp/receiver, this option is to choose whether it is going to output DD and DTS along with PCM. You will either get an output or not and it will make no difference for PCM.

If you are using the analog outputs to the preamp/reciever, the setting is irrelevant.
I believe PCM is the default for Audio, 44.1khz/16bit.

While Bitstream is a multichannel (Dolby Digital or DTS) format available from encoded Discs, such as movies or concert DVDs.

Just guessing, but, I bet if you select PCM only that the DVD player will downmix DD/DTS(multichannel) to PCM(2 channel). Select PCM and put a movie in and see what happens. Is it 2 channel only (stereo)?

On my DVD player, I use have bitstream selected with the highest output. 96khz/24bit methinks. When I playback an Audio CD it always defaults back to the PCM standard of 44.1khz/16bit.

If you are going to use the DVD player for movies and concert DVDs in a multichannel system I suggest Bitstream. If you have a 2 channel system only then PCM is fine.
Ha! In light of subsequent answers, mine shows just how familiar I am (not!) with multichannel, HT, pre/pro's, and all that jazz. Maybe one day I'll even buy myself a DVD player and join the modern world...Right now, I still think my naivete is more charming than it is pathetic... :-)