Surround sound without Optic cable?

Hi. I found this forum. And i thought maybe you someone could help me. So i got the surround system from my brother. It is quite old and it doesnt have an digital Optic input. But it has something like CDR digital IN(and out) It kinda looks like an ordinary RCA cable. So if i gues correct the CDR Digital Input should be able to have surround. So i wonder if there is somekind of an adapter or something. Like CDR Digital to digital optic or something. And could someone tell me what the hek is CDR digital Input. Because i am avare of digital optic cables but i never seen a CDR Digital cable. And i googled it and its like there is not such thing. So the main thing is how can i get 5.1 surround without digital Optic input. As i say i have CDR digital INput.

Thx for your answers...