How can I get my new receiver to decode the AC-3 s

I have a laserdisc player with an RF AC-3 digital output. However my Sony DA4ES will not output sound when I make that connection and force it to use it. Is the receiver incapable of decoding the AC-3 signal? I just assumed, being a close relative of Dolby Digital, that it would "get it". The receiver properly decodes all surround formats output from my DVD player. Any thoughts would be appreciated.
I believe you need an RF demodulator between the AC-3 out from the LD player and the input to the receiver. Three that are fairly reasonably priced, used, are sometimes available on ebay. They are:

Yamaha APD-1
Pioneer RFD-1
Kenwood DEM-999D

The Yamaha is probably the first choice. They sell for around $120-$140.
Dolby Digital is AC-3 (and vise versa). I agree with Ncarv that the problem is not the receiver, but probably the Laserdisc player RF signal is not the same as a DVD player signal and therefore not compatible.

Are you sure your Laserdisc player will read Dolby Digital?

I will check with a videophile friend who knows a lot about laserdiscs. He has owned all the variations in laserdisc technology going back to the original Pioneer analog players. I have his old first generation digital laser player, and I've never tried to run it other than in Pro-logic.
The previous two posts have correctly identified the problem as being the RF modulator. If your receiver does not have an RF-modulator input jack for laser disk players (and most new units these days won't, since laser disks are "old technology"), then you will have to buy one and insert it between the laser disk player and your receiver. Laser disks used an analog recording format for the audio signal, not Dolby Digital (or other digital surround medium), which is why the audio signal must be converted to a radio frequency that can be utilized by the receiver or surround processor.
many of the newer model pioneer elite receivers still have built-in rf demodulators. i use the vsx-37tx in my HT system, and i know for sure it has one, since i use it for DD 5.1 from my LD. . .

check the features on the website, it'll say: ac-3 rf demodulator or something similar. or you can purchase an external one, as the others suggested.
sdcampbell, just fyi: you are incorrect in stating that ld's do not use dolby digital or other digital formats - it depends solely on the disc. many older ld's have an uncompressed pcm track as well as an analog-only track. and many newer ones *do* have DD and DTS tracks, along with a pcm and an analog track. they (the DD and DTS tracks) are indeed RF modulated and need to be demodulated in order for a receiver to recognize the signal, but that is because the commonly accepted technology changed for sending compressed digital data from rf modulated to using a digital cable with the popularization of dvd.
the standard analog tracks are not rf modulated, because they don't need to be. the go out via rca jacks like anything else. the pcm tracks as well would be decoded by the player's internal DAC and then sent out the rca's.
Get it modified by MSB
I am back... My friend sais you will need the RF adapter. He has a Kenwood 990D player that has a coaxial output like a DVD player (also the RF). So he does not need the adapter. Pioneer made a few later players this way also. If you don't want to spend the $100 for the RF adapter you could look on eBay for another machine.

Also Pioneer currently makes a DVD/LD combination player. The regular DVD coax output works for laserdiscs also.
Thank you to everybody who responded, you have all been most helpful.
For future reference, this is a great explanation