[BioC] Phred encoding

[Taylor, Sean D]

Hi,

I'm trying to quality filter my NGS reads and want to filter out reads that have bases below a quality threshold (say 23 for instance, using Illumina MiSeq with an offset of 33). Can anyone tell me why the results of the two functions as.raw() and as.numeric() give different results?

qual<-PhredQuality(c("BBBBBFFB4!"))
as.raw(unlist(qual))
as.numeric(unlist(qual))

Thanks,
Sean