Shares of RealNetworks ( RNWK) were falling after the company said it wants to offer about $100 million of convertible subordinated notes to institutional buyers. The zero-coupon notes would be due in 2010, and the conversion rate and offering price will be determined through talks between RealNetworks and the initial purchaser of the notes. Convertible deals often drive share prices lower initially, and that was the case Wednesday with RealNetworks, whose stock dropped 40 cents, or 5.4%, to $7.03. Investors tend to worry that the conversion of debt into common stock will dilute future earnings. RealNetworks also said it expects to grant the initial purchaser of the notes an option to buy up to $10 million more of the debt. The digital media company plans to use the proceeds of the offering for general corporate purposes, acquisitions and working capital requirements.